Stumble at high RPM, popping, and cool header

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Tyboy11

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Hello all,

Well I finally started to troubleshoot my 85' Max (29,000 miles) and could use some advice. It was great a year ago when I bought it, now it gradually got to this point. Bike is completely stock except drilled muffs.

  • It hesitates at WOT. (when i'm expecting the boost to kick in, hesitates/catches, just not all there consistently).
  • It also pops when I decelerate.
  • hard to start sometimes.
  • The #3 cylinder exhaust at header is almost 100 degrees cooler than others after a ride.
  • Runs ok at other RPM's.
  • Plugs look normal and show to be firing good. (#3 has black residue on threads near end and lip facing electrode, electrode and insulator clean)
  • Pulled #3 plug after ride and it had no residual fuel on it. (did not shut off at WOT and check)
  • Could not find any Vacuum leaks.
  • Just pulled carbs today, float levels on #3 and #4 are good.
  • #3 Cylinder outboard valve has excessive carbon build up (all others spotless).
  • Has a fresh fuel filter and fuel pump.
  • Fuel tank is not rusted out.
  • #3 and #4 bowls looked clean after popping cover today.
Also, I sync'd carbs awhile back (no change). I have been running seafoam for awhile (no change). Shotgunned and peashooter was done awhile back (no change).

I'm getting ready to pull jets and see if they're clear, but that valve and the fact that the #3 exhaust is so much cooler than the others has me a little freaked out. Since I don't have any residual fuel on plugs after shut down, I thought my coils, wires and plugs are good.

If it was my coil not getting enough spark at the higher RPM, wouldnt my plug show signs of being rich?

What does this indicate to some of you out there with experience on these bikes?
What would your next step be?:ummm::confused2:


Thanks

TYBOY
 
A compression check would be a quick way to RO an internal problem. I had a few hassles with HT leads before I converted to COPs so maybe a peek at the HT leads and plug caps just to RO any issues there. When were the plugs last swapped out?

Might also use a long screwdriver to gently push the slides back and make sure none are binding (engine off), and they all move about the same when you blip the throttle at idle.
 
Thanks for your response,

I actually checked all the diaphragms with a screwdriver prior to carb removal. I pulled the #3 out prior and everything looked good, no holes, no binding etc. Tomorrow I will look at voltage going to the coils ( I have to read up on that ) and report. I am wondering if maybe the coil is lacking at higher RPM and not burning off all the fuel, only to burn it off eventually when i decrease throttle.

How would the compression check help me? Would that show if a valve was sticking? I'm open to all suggestions!

This has got me baffled!
 
Compression check just to make sure no ring wear, or valve wear/adjustment issues causing low compression/incomplete combustion. Its a quick check for piece of mind. Between the Vmax forum and Venture forum it seems internal engine problems with Yamaha V4s are a bit less common than ignition and carb issues but it never hurts to rule it out before throwing a lot of effort and money elsewhere.

Do you know when the last valve adjustment was? Since you said the onset has been gradual and nothing you have tried has effected it then a compression check would be handy to make sure your not chasing something deeper. Ive not had it happen on a Vmax but on other engines I have seen valves so far out of whack that compression and drivability have been effected. When I find myself troubleshooting issues like this I find myself instinctively reaching for the compression gauge.
 
I have no history for the bike, so I would assume the valves have never been checked. I have a oil leak at the #1 cylinder valve cover, so now is a good time to replace gasket and verify valve clearances.

I will try and come up with a compression tester somehow. I wonder if Auto zone would have one for that size plug?

Thanks again for your help!
 
Best of luck. If you dont use one often you can skate by with a less expensive model, i have a cheapo at home, I think it was $30 and came in a hard case with several adaptors to fit about anything I need to test. I have used it plenty on everything from mowers, saws up to bikes and cars. Autozone may even have loaners, I heard they used to, or still do that. It would suck to buy one to use it only once but I dont think I could go through life without one because I do my own vehicle work. If this is a bike your going to keep and put some miles on then the VC gasket and valve clearances should be addressed anyway but I would hate to send you to far down that road if your focusing on this issue right now. Im trying to imagine what else could go wrong with a carb to cause this gradual onset of symptoms and nothing comes to mind that you havnt looked at already. I know we have some really knowledgeable folks here so maybe we could get a second opinion on a compression check being a good next move for this issue.
 
UPDATE:

Well I was checking out the ignition system today and found the probable cause. The primary on the coils checked good, but when I checked the secondary on #3, there was no Ohm reading at the spark plug cap. I pulled the coil and checked the coil itself and readings were within tolerance. I put the wire back on and readings were intermittently in and out. I cut a 1/4" off the wire going into the coil and re-stabbed it. Readings are now within limits on the secondary check with the wire installed.

I still have the carbs out and separated and am waiting for my float bowl gaskets. I am still going to pull jets and clean the carb the best I can while it's off.

I feel like a jackass not checking that first, but i'll finally be able to check condition of the internals of the carbs.

Lesson learned and if this does fix it, I will update post so others can use this.



Thanks alot for your responses!


Tyboy (with fingers crossed)
 
Fingers crossed on your behalf. These plug caps are older than shit and known to fail. You can get new ones from NGK for about $3 each, IIRC Autozone sells those and the universal NGK wires that trim to fit. If you go this route let me know and I'll find a writeup someone on the Venture sight did on replacing these with part numbers and all. The only difference IIRC is the new caps are 5k and OE are 10k ohms and I dont recall that being an issue. I hope thats the issue!
 
When I put carbs back in and reinstall coils, I'm going to swap #3 cap and wire with #1 cap and wire for the test ride. If it still runs like crap at the WOT and the cool header has swapped to the #1, then I'll know I still have an issue with the coil and wire.

If that's the case, I may just go with the COP mod. Seems like a cheaper alternative than $83 a coil on bike bandit.


I'll keep updating this post for others future reference.


Thanks again,


Tyboy
 
It cost me ~$30 and a couple hours to make a set for my Venture. Getting your rear Vmax coils out is pretty straight forward but removal of the fronts is a motherfucker. I understand why some folks leave the fronts in place after going to cops. I'm OCD and they had to come out! I bought a set for the Vmax but had to remake them. Its really not hard to do.
 
If the carbs are apart and on the bench, I'd buy some jet block gaskets and pull the pilot jets just to make sure they are clean.

The OEM spark plug boots can be disassembled. There are pictures of it floating around on the site. It may be worthwhile to clean those up.

Bulk copper core plug wire is cheap and readily available

Sent from my SM-G930V using Tapatalk
 
If the carbs are apart and on the bench, I'd buy some jet block gaskets and pull the pilot jets just to make sure they are clean.

The OEM spark plug boots can be disassembled. There are pictures of it floating around on the site. It may be worthwhile to clean those up.

Bulk copper core plug wire is cheap and readily available

Sent from my SM-G930V using Tapatalk


Agreed, jet block gaskets and other packings are in route. I ordered 4 of the K & L carb kits. I'm going to pull jets and clean up the carb the best I can before re-installation.

I'll look around for the boot disassembly and plug wire info.

Thanks,
 

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Got my 4 K & L carb kits part number WP-118-2879 and 4 ebay carb boots part number JKJ-28. Thats enough to tear into the carbs this weekend.

Looking forward to it! :boxing00:
 
I went thru #3 and #4 carbs today.

These are my jet sizes. PAJ1 90, PAJ2 177.5, Main jet 152.5 all Mikuni stamped.

After I was done with those two, I pulled the jet needles out of the diaphragms to take a look. I noticed that there was one small washer under the spring. Does the stock configuration have one washer or did someone add this to richen it up? Should it just be the spring going on the plastic base at the needle?:ummm:

I also learned that Purple Power will tarnish the chrome diaphragm covers. I had them in an ultrasonic with other parts. I was using Purple Power and water 50/50 mix. It turned them dark with streaks. Looks like shit.:bang head:

One more thing: When I set my floats tomorrow, can I set the distance with them upside down, with the float resting on the valve or should it just be depressing the tit for the distance (not with the full weight of the float on it). I've seen two different ways on this forum.
 
Post a pic of the needles themselves. it will be easy to see what has been done to the carbs (Mods done by the previous owner)

PAJ#2 SHOULD be 170, NOT 177.5. Double check that just to make certain.
 

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