Ok Hawk gurus, what is my problem??
 

Hey guys, been reading for years, never had a reason to post, till now. I got hurt bad, so my 98 Hawk sat for 3 years. When I rolled her out, all the typical stuff was bad, battery, fuel lines, vacuum lines, etc. Pulled the carbs, freshened them up with new honda orings and cleaned them out, although they were spotless in spite of the situation. While it was apart, put some manual CCT's on her (20k miles).

Well, it starts immediately and runs perfect, for a few minutes. Anything more than that, and the front carb starts backfiring, spitting, hissing. If this is someones video, sorry for using it, but this is EXACTLY what it is doing, even same cylinder.



If I put my hand over the rear carb plenum it sucks HARD. Hand over the front, pretty much nothing. Eventually will slow idle or rev. I know it acts like valve issue, but it isn't as far as I can tell, see below. It also runs much better, almost right with choke on, therefore possible lean issue?

I'm not a greenhorn mechanic, 20 years master ASE, but I can't figure this one out, anyone have any ideas? So here is what I have done/checked.

RE checked timing, both cams are spot on.
RE checked valve lash, middle of spec.
Compression SOUNDS good, have not checked with guage, will on weekend.
Removed carbs and checked boots, seating, and install, 3 times.
Re check and clean jets, all clear.
Re set fuel screws to 2.5 front 2.25 rear.
Vac line on rear, at vac diaphragm port.
Carbs synced.

I have found many many vac leaks with a can of starting fluid, if you are a mechanic you know what I'm talking about. Cant find anything at all, sprayed all around the boots and around the carb. I'm willing to try anything, but I don't know what else to check! Remember it runs great for the first few minutes! Also, last time it was used for daily commute 3 years ago, it ran flawlessly so it isn't worn valves or something like that. What do you guys think?

Bad diaphragm? And I'm just winging it based on what you've eliminated.

Alan

Also just winging it; if it starts fine and runs for a few minutes then it won't be compression. Much more likely to be fuel supply related I would guess. The vacuum petcock is a good place to start, and maybe also the fuel strainer on the tank outlet, or the tank breather hose.

Are you running it with the tank on? When it "runs good" for a few minutes, does that mean it idles or that you can rip down to the store for yogurt bars?

I ask because the symptoms sound fuel related. It sounds almost like you don't have the vacuum line on the wrong part of the petcock.

Everyone knows what I'm about to say but........If you haven't drained the tank to empty and swished and cleaned it, then whatever junk is in the tank will ruin your life indefinitely. So start there. Many woes are solved by that alone. (use brand new premium gas to refill).

Its also smart to put some small inline filters on the 2 gas lines.

Does it run better with the choke permanently? Meaning if its fuel delivery, the choke would simply mask it by doctoring the air fuel mixture. So I am saying you may be getting a trickle of fuel but not a flow.

Plus you may have gunked your carbs with gas tank crud and be blind to that since you just "cleaned" them.

My first thought is the diaphragm!

Second is fuel petcock,

3rd is incorrect vac line hook up

4th is ?

Please let us know what happens.

Good luck

Good call with 1,2 & 3.
The symptom that doesn't match fuel starvation is the lack of suction on the front cylinder. If the engine is spinning and the valve timing is right, there will be air being pulled in. So is this #4?
Did you start and run the engine before the carb refresh and more to the point, replacing the CCTs?
The hissing and spitting seems like a timing issue to me. $.02

Plugged fuel tank vent? Causing vapor lock after running a bit..once it runs bad, open tank cap with second key.....

Petcock diaphragm leaking, not opening, not flowing enough fuel....as soon as it starts running bad, clamp or remove tank fuel lines, drain carbs into two clean containers, compare amounts.

Electrical issue after heat soaked or at Op temp...coil degrading under heat? once it runs bad get a header temp just off the head,,,,do you have one cylinder not firing? if so, check the plug, plug wire, coil wiring...clean the ground terminal and the mount/screw, check the positive connection as well. If you consistently have one cylinder drops out after motor heats up swap coils and see if issue follows, or swap in a known good coil

Sorry I have been out for a few days! Thank you all for the replies! So to answer a few questions that I should have listed in the first post, I removed the tank and disassembled, cleaning the tank, petcock and filter well before initial re-assembly. I have NOT checked the petcock diaphragm tho, I will do that. As for the CV diaphragm, they sound good, as in they produce vacuum when the slide is moved. There was one little rip in the top rim, not of the round part of the diaphragm but of the small oring that comes off the side. I didn't have a new one, so I siliconed it on re-assembly. Maybe that is part of it!.

To answer length of time, it runs enough to warm up.....it starts and idles super easy as always, but by the time I back out and start down the driveway, its doing this popping and spitting and I quickly loose the front cylinder completely. I have not run it very long with the choke on, only long enough to look for leaks and mess with it a little. Even with the choke, its not perfect, just better.

The vac line is on the vac port on the back, not the vent on the bottom.

I agree completely that it looks like a symptom of bad timing! It had not been started, all work was done at the same time. Problem is, timing is on?? It was done properly, with covers off, timing marks aligned, all that. Also, I have since removed the valve covers to check, again, to make sure marks are aligned. I cannot figure out in my head how it is possible to have a lack of suction IF as you said, the engine is turning and timing is correct. Maybe there is a bad air leak that I just cant seem to find. Strangely, the vac ports on front and rear are actually producing very much the same vacuum, around 14 in. Also, why would it run great for 5 min? If timing was off, it wouldn't run well ever it seems. Really strange.....

I will look into the electrical side, checking header heat etc. Maybe I'm loosing a plug?? I will look into the diaphragms, for both CV carb and petcock. Its a great idea to check bowl volume, I will do that as well. I thank you all for you thoughts, Ill put them to use and see what I can find, I'll keep you all updated!

Site isn't posting my replies, is there some setting that I might have wrong?

Ill try again, the whole freaking thing! Sorry I've been out a couple days. So, to answer a few questions, the tank, filter and petcock were removed and cleaned before I ever tried to start it. Also, all the work was done all at the same time, I only tried to start it once after sitting with obviously no luck. So I know that the tank, filter, petcock, lines and carbs are clean and un-restricted.

The vac line is on the vac port on the rear, not the vent on the bottom.

As for the condition of the petcock diaphragm, it seems to operate, but I have not disassembled it, only checked it with vacuum (holds vacuum). I will check the actual diaphragm and see what its condition is.

For the CV diaphragm, they seem to operate ok, and sound good. Both slides move at the same speed, and I can hear them "breathing". I also visually checked them. The front carb does have a nicked spot on the top of the diaphragm, not the round part but the little oring that extends from it. I sealed it with silicone becuase I didn't have another one, but maybe that is leaking some? I will check.

It runs for about 5 minutes. It starts great, idles nice, and revs good. But as the 5 min progresses, it gets worse and worse, eventually I loose the front cylinder all together! Choke HELPS, it doesn't make it run perfect. But it does help, seems to rev with choke on. After the 5 min, without choke it wont rev or really even run at all.

I agree it looks absolutely like timing issue, but I really dont believe it is. One, it runs perfect for 5 min. If timing was off, it wouldn't run. Second, I have re-verified timing several times. Cams are dead on. I cant figure how it can be on, and engine turning, and not suck as Xeris said. Must be a leak that I just cant seem to find! Strangely both vac ports are pulling roughly the same vacuum, about 14lbs at idle or a little above.

I will check electrical. Both pipes are hot, but I have not shot them with IR yet, I will do so. Maybe I'm loosing a plug, I'll swap coils and see. Also, great idea to check bowl volume, that would tell me a lot about fueling restriction! Thanks for all the replies, I sure do appreciate it. I will put all the suggestions to use, and see what I can find! I will keep you all updated! HOPEFULLY THIS ONE ACTUALLY POSTS

Clean the front carb at least again after stripping it down bare and using compressed air; check the slide diaphragm and choke fitting for cracks/leaks.

Recheck valve timing and other things mentioned but I think the carb is gunked up and maybe the petcock and/or strainer has issues from crap in the tank. I run the fuel filters from the Honda GL1200...

Mine done the same exact thing, I even pulled the head because of it.
Same stuff you are going through.
I got fed up with it and parked the bike got so frustrated dealing with it.
Eight years later I decide to rebuild the bike get it going again. In the rebuild i found a small crack in the front coil leaking voltage, but also found that the fuel valve diagrams were bad and leaking not allowing them to open freely. Though they don't appear torn or have visible holes they were bad. The front cylinder was actually sucking the float bowl dry. I know it sounds crazier than hell, but I replaced the fuel valve not rebuilding it, and problem solved.
I'm not 100% guaranteeing the fuel valve to fix your problem, but something to try out of curiosity is to fill your tank up completely and ride the bike see if it goes further before dropping the front cylinder.

Hey guys I don't know if anyone is still following this or not? I have been overseas for quite some time. Anyway, I am back and able to get to the bike!

I replaced the intake boots, and the problem instantly vanished! They were delaminating, and had been leaking. Super strange that vac tests and starting fluid spray around the outside revealed nothing at all, but when I took the carbs out this last time, they were clearly bad.

So I took her out on a testdrive, and now I have ANOTHER problem. Bike runs as smooth and perfect as it ever has, under 3/4 throttle. I thought it was totally fixed, so I went to pull the front wheel up a little, and it totally fell on its face. Sounds like a rev limiter kind of stumble. I don't think its missing, its strange. Back off the throttle a little, and it shoots off like a rocket. In neutral it will run smooth and clean to redline, and under less than 3/4 throttle will do the same out on the road! Say like if I'm going down hill and doesn't take as much throttle, it will go any RPM smoothly, any speed smoothly. It has absolutely nothing to do with RPM, it is 100% throttle position. And I mean MINUTE amounts. If it begins to miss/stumble I can back off the tiniest amount, and it clears right up and takes off. Again, idles perfectly, and runs completely normal, under 3/4 throttle.

Seemed like fuel delivery, so I checked all the hoses and the petcock. Valve was acting a little inconsistent so I rebuilt the fuel valve and it works properly. Charging system is at 13.8 or so. I took it out and got it "stumbling" and played with the choke. It doesn't respond at all to the choke, in or out, it doesn't help or make it worse, so it doesn't seem like fuel.
Because of this I re checked the TPS. I had previously reset it to 500 Ohms as most say to do. So just to try it I moved it back to around 800, and it HELPED considerably. But it is still doing it! Before the TPS to 800, it sounded more like a complete miss, it would nearly die. Now at 800 it sounds more like a rev limiter, but again it is not related to RPM only to throttle!

Is something wrong with my TPS? Ignition timing? I think I will replace my plugs, just to try it, but they are not very old. I want to stress, this bike ran perfectly from day one till I got hurt and put it up, so I know the history on it. Its never done anything like this before, except a few times when I was riding in the rain, it completely quit, just like hitting the kill button. Sat for a good long time, and then it was fine. Not sure if theres any relation to that. Any ideas?

Not the plugs, but have you looked at them? Might help point you in the right direction.
Carb diaphragms?

Yes and main jet or jet needle, if fuel valve fills the bowls.

check bowl fuel level and cc's not just inverted float setting

I have cbr600 stick coils

Thanks for the replies guys!

I have not looked at plugs yet, I will pull tomorrow and see how they look.

Is there any way to test the diaphragms? Can I block the passages off and vac test them?

I will drain both bowls and see what volume I get tomorrow as well as check with an external pipe. What should the cc and fuel level be, I don't have a manual?

One more question skokievtr, why the CBR coils, did you have problems with yours? It really acts like electrical, but is directly tied to the throttle. I assumed if it was jetting that pulling the choke would clear it up temporarily, that's why I was thinking electrical, but I know how these carb issues can get real weird sometimes.

When you set the TPS, did you operate the throttle through the full range and observe if the increase in ohms was smooth, as in no jumps or dropouts?
Not sure if this would make any difference. The ignition timing is adjusted based on the signal from the TPS.

Yes, TPS is smooth no jumps or hangups.

Plugs look fine, a little black because they haven't been hot at all, I can only putt around for quite some time now!

The front carb has less fuel than the rear, I drained once on bike and got 38 ml front and 46 rear. Refilled by running for a bit and then pulled the carbs and carefully drained directly into a beacon. That time I got 45 rear and 42 front. Much closer, and I feel much better about the second round, feel like it was much more accurate. Not sure if such a small amount would matter? 38 to 46 is definitely a problem but I'm not sure that was accurate.

Anyone have specs for bowl volume or float levels?

So you can set the float level to equalize them. I still wonder if its a vacuum issue that is not delivering fuel.

Did you remove tank and put vacuum to the petcock to see if fuel flows? That's the issue that would cause those issues all day long.

The float level should be 16.6mm but is not adjustable.

Hmmm. Seems like this is telling you something

I know it's been discussed ad infinitum, but I had a similar problem with my '98 shortly after I pulled the tank and carbs for jetting. After reassembly there was no firing at all on either cylinder. Yup, you guessed it vac. line on the nipple on the bottom of the petcock. Which is the diaphragm vent. With the help of this forum, I corrected that. Ran fine for awhile then started firing on the rear cylinder only. Checked the spark, OK. Checked petcock by applying suction to vac port, seemed okay. Pulled the petcock apart found a minute hole/crack in the diaphragm. Not big enough to cause valve failure but big enough to apparently not allow the valve to fully open. I still don't know why it only got enough fuel to the rear carb but, after replacing the diaphragm. the problem was remedied. Just a thought.

First, thanks a lot guys for all the imput, I really do appreciate it. It sucks having this thing down for so long.

Cadbury64 thank you, I saw a page of the manual which said to replace floats if not in spec, thank you for the spec I will check them. Sure seems to be to be that if its anything in the carb. I pulled the carbs apart again just to check, and everything is spotless and clean. No plugged ports anywhere. Also, the only way that I can see that the choke wouldn't make a difference while its sputtering is if it is fuel supply BEFORE any jetting. If the bowl was drying up, then choking it wouldn't make a difference. Same with petcock issues, lack of fuel would cause that. If it was jetting plugged, the choke should have corrected it I'm thinking?

CaryDG I did rebuild the petcock again, and checked the tank screen. All looks good, holds vac, and flows freely, at least while testing. Not saying it isn't doing something goofy while running, and sucking one side dry like you were saying.

Xeris I'm with you sir, changing the setting definitely made a huge difference! What kind of electrical problems could I be facing? Do I have a coil crapping out I wonder?

I am going to reassembe carbs, I did the slide diaphragms as they were a little dry on the top seal lip. I'll test again, maybe one was sucking air there. I'm really wondering about electrical tho, I've never seen any fuel problem be so sensitive to the tiniest movements of the throttle, but I haven't worked on these CV carbs much. Thanks guys, I'll report back.

The guy that owned the bike before me removed all the PAIR system, the head fitting/reed valve just had a rubber plug over the fitting. It was leaking. I am going to machine some blank covers and get rid of the factory fitting with the hose connection on it. Could that leaking cause any weird issues like this?

Also, while the carbs are apart, what would you guys recommend for carb settings? My jets are marked 45s and 190. Pilots are 2.5 turns out. It has full exhaust and carbon cans, and a K&N (yeah, I know, but it has always been great since new) do you think I should change anything while in there? Is the K&N any good if jetted properly, or is the bike just better with the factory filter no matter what?

For jetting we kind of need to know where the bike is.
As for what you are running, I posted a thread on my opinions on the matter. =)

Duh! Sorry wasn't thinking, I am south of Tucson, 4500 feet and always dry and hot.

​​​​​​​I will check out your post thank you!

That might put you in Sierra Vista? Or the town too tacky to die!

The K&N is an unknown (for me) as the main jets are concerned. With a stock air filter 190 would be big. I'm running a flow commander and have it lean of the mid point by a full turn+(?), don't have my notes in front of me. I ride mostly at 4800- 5600ft. When at 4500 and below the bike is noticeably zippier
The conventional wisdom (here), is to put everything back to stock and start from there. That does not include putting the PAIR back on.
Read Hawks carb sticky. That might help in the overall understanding, but I don't think that his tune is exactly right for the elevation that your at. It would be a good starting point.

I didn't even notice your location, Bisbee thats awesome! Yes I'm in good ol Vista ...... no further comment haha. I ride (or used to) to Bisbee frequently. We will have to go for a ride some time. I used to like to go to the Screaming Banshee, haven't been there in a long time, don't even know if they are still there.

I'm probably not going to mess with jetting much, I can't imagine that it would even run with whats been done to it if it were stock. It has always ran so good, I was just curious what was recommended. That carb sticky is great, I am going through it.

As a note, the carb floats are set perfectly, within a 64th of spec. Bowl volume is very close, I wonder if the slight difference is from the carbs sitting slightly out of level when on the bike? Anyway, its back together. I did the slide diaphragms, and double cleaned everything. Going to try again. If it persists, I am convinced it must be electrical. Something related to the TPS, coils, or something like that.

Cant wait to get it going so I can get to machining the flywheel down, making a rear suspension spacer and all the other good mods I read about here :D

SOLVED:

So she is finally back. After much tweaking with the electronics, I ruled them all out. The change I felt must have been due to some other factor, hot/cold/ambient temp, etc.

I noticed that while the air filter was off and in neutral, that when I whacked the throttle, it stumbled and sounded just the same as the stumble while running. So I stuck the filter back on, the air box cover and whacked the throttle in neutral again and it sounded great. Got me to wondering about this K&N. So I kind of over-oiled it just to see what would happen, and the problem was worse. So I blew all the oil off the filter with an air nozzle till nothing more would come out of it, and VOILA! I couldnt believe it. These bikes are so stinkin' sensitive to tiny things. Its running really well, but I'm thinking I just need to put a stock filter on it, from what I've read here.

I want to thank everyone for the help, I love this site I read much more than I contribute, but you guys are great.

So to summarize, the first strange problem was a vacuum issue, caused by a bad carb boot. That was causing the issue in the video above.
The second problem was simply a little more oil on the filter than it should have had, causing a complete choke out over 1/2 throttle or so.
I definitely learned that these bikes with these massive carbs can really be temperamental!