Engine starts to miss above 5000 RPMs

[mnferwerda]

Over the winter I swapped my oil leaking/burning engine for a similar engine (@18,000 miles) that I bought off eBay. During the swap, I also replaced the petcock (Suzuki) and the spark plug caps (NGK VD05F Resistor Spark Plug Cap). I checked the valve clearance and had to change a couple of shims but none were way off.

Got it all together and it started right up. I had to adjust the idle setting and the pilot/air screw jet setting just a little bit. Synched the carbs with the my new CarbTune pro tool (that thing is sweet!), and took it for a ride. Pulls good from low RPMs, but seems to be a bit hesitant to get to the redline (8500 RPMs). What I am really noticing though is that when I'm riding at about 65 and above for a couple of minutes, the engine starts to miss. If I give it some more throttle, then it will start to pull...

So I'm trying to figure out what the culprit is. Just put in a new petcock so that should not be an issue. The previous engine had no issues at that speed so I would think the float level should be OK. Could it possibly be that I need to go up a size on the main jet? I have 137.5 on there now (originally had 132.5). A more general question would be: Would similar engines require different jet settings for peak performance? I sorta thought no, but I'm beginning to think I'm mistaken.

The other culprit might be the spark plug resistor caps. Of the old ones, one was original, and the other had a cracked weather seal, so I thought it was prudent to replace them both. The bike had no issues running at 60, so I'm inclined to think that this is not an electrical issue.

A bit of a ramble here... I appreciate any thoughts you have.

Thanks - Mark

 

[tom203]

"the engine starts to miss. If I give it some more throttle, then it will start to pull..."

I'd go back to original main jet size....might be too rich loafing along ..until you open throttle more.

 

[chuck hahn]

When it hits the dead spot try some choke..If that makes it go away your lean on the top end. If not ( or makes it worse) your rich.

 

[Spyderman]

Got it all together and it started right up. I had to adjust the idle setting and the pilot/air screw jet setting just a little bit. Synched the carbs with the my new CarbTune pro tool (that thing is sweet!), and took it for a ride. Pulls good from low RPMs, but seems to be a bit hesitant to get to the redline (8500 RPMs). What I am really noticing though is that when I'm riding at about 65 and above for a couple of minutes, the engine starts to miss. If I give it some more throttle, then it will start to pull...


Thanks - Mark

speed and RPM have nothing to do with jetting
your jetting is still not correct
throttle position is the only thing that will tell you what area is not correct

idle to 1/4 throttle pilot jet fuel screw & air screw Vm carbs or mixture screw on the CV
1/4 to 3/4 throttle jet needle and needle jet
3/4 to wide open throttle main jet

doing plug chops and then removing the plugs to see what colour they are will tell you what is happening and if you do a search you should find a thread or 2 that will tell you how to read a spark plug
and that will along with the throttle position will tell you what area you need to adjust to remove the miss
an educated guess tells me that your pilot circuit is what needs adjusting but if your main jet is out all other areas will be out as well
also start with the main jet and then work back to the pilot
make one change and test it see what it does then do another and so on until it is right if you make more than one change at a time your never going to get it right
hope that helps you go in the right direction

 

[zuluwiz]

Look for air leaks in the carb boots.

 

[TxGSrider]

Does it "pop" when the motor starts missing or does it just kind of "lay down" and bog?
Also take note on how far the throttle is open when it starts to miss and how far when it picks back up. That will guide which circuit you should be working on.
If its acting up somewhere between 1/4 throttle and gets better at 3/4 throttle, its probably going to be needle position.

 

[mnferwerda]

Finally got a decent enough day (low 40s and no rain/ice) to try out some things. Throttle is at about the 1/2 way mark when it starts to miss, no popping heard. It goes away with the choke full on. Engine will not pull beyond the redline (8500 rpm, choke off). I didn't do plug chops but when I pulled them to do a compression test (it is a "new" engine after all, and I wanted to get a baseline), the plugs were white.

So I'll start with the main jet and go from there. Thanks for all you help so far.

Weather looks awful for the next week...

 

[chuck hahn]

And there you are white plugs mean lean and choke makes it go away just as I said it would.

 

[Steve]

Don't forget, ... when you are changing jetting and other settings, CHANGE ONE THING AT A TIME.

For example: if you up the main jet size, shim the needle AND add about 1/2 turn to the pilot screws, you won't know which one of them actually helped. In fact, one of them might have helped A LOT, but the other two hurt just a little, but still with a net gain. If you change more than one thing, you simply won't know.

.

 

[mnferwerda]

I upped the main jet from 137.5 to 142.5. Seemed to run better till I reached the end of the hwy, did a U turn, and redlined it in 2nd, 3rd, and 4th and not quite in 5th (there's this speed limit thing...). Then it started missing a lot. I'm beginning to think this is not jet related. It would seem jetting would cause a rich or lean condition, not a missing condition. I'm thinking I'll put the original petcock back on and see what happens.

 

[chuck hahn]

Out a timing light on each plug wire and run it up and watch for intermittent or a full cut out of flashing from the light. I got a Honda right now Im working on and it was dogging after 4500 and struggled to get anything higher. Tried everything and the light showed me that the signal would stop for a full 3 or 4 seconds on the 2/3 coil. Swapped in new coils all around and the problem was gone.

May also check timing and be sure the advancer is working correctly.

 

[mnferwerda]

I got out my timing light that I have NOT used in years (and considered getting rid of) and hooked it up. Started the bike, looked at the light and saw occasional misses at idle on #1. Switched to #2 cylinder and saw more misses at during rev ups. The obvious thing to check was to check the spark plugs cap that I had installed over the winter. They seemed good but I redid the install. I took it out for a ride and problem solved! Sweet!

Thanks for the ideas!

Of course, now the gas gauge starts acting up. That's a bit easier to debug...