Slight popping

[jsandidge]

I am trying to get my bike road ready. It runs fairly well now, but I have yet to put it on the road. (Still needs tires, and I'm not finished with my brake rebuild).

Here's what I've done so far:

Oil & flter change
Strip and dip carbs with new o-rings
New intake boots and o-rings
Clean and oil air filter
Weather strip airbox
Valve adjustment with new cam cover gasket
Vacuum synch carbs
New plugs & plug caps
PO installed new Nippondenso coils
New right hand controls
New battery
Bypassed clutch safety switch
New exhaust connector gaskets

The bike idles well and throttle response is good. It does not smoke. The problem is that it pops from the exhaust when I rev it to about 4000 rpm. This happens whether I hold it at 4000, or if I rev it up and let off right away. It will rev higher, but I've never held it at higher revs.

I have confirmed that there are no air leaks by spraying starting fluid all around the carbs, boots, and airbox. There are also no exhaust leaks, and everything is bone stock.

Does anyone have ideas as to what I should check? Thanks in advance.

Drew

[Crankthat]

Hello.
I do not have the experience as some of the guys here but willing to try and help.
I think you are in the right direction looking for air leaks.
Possibly recheck with starter fluid while somebody else is holding the throttle open and revs steady in range where problem exists.
The possibility exists that a jet once cleaned and holding at idle is re contaminated with a spec of dirt of something?
The hose from top of valve cover to airbox fastened well?
If you cannot find the problem right off when you have the bike otherwise roadworthy find a infrared thermometer check pipes at idle (temp) look for difference in temps coming off the head.
Try again after riding bike at problem rev range.
There will be some difference in temps you are looking for the one with the biggest difference.
If the problem lies in the carbs that will tell you which carb has the problem.
I hope this helps you.
P.S. you may also find the carb culprit if that is where the problem lies by pulling the plugs and comparing.

[1_v8_merc]

Carb mixture screws might need to be dialed in.
Or you have an exhaust leak.
I can't recall how many people "thought" their exhaust was airtight, but was actually leaking. lol. Just double-triple check it.

[tom203]

Make sure petcock vac hose is connected (and not leaking either in hose or at petcock itself). I'd disconnect it and plug #2 carb vac fitting to test things- run on "PR" spot. This lets you rule out petcock as a possible problem.

[jsandidge]

petcock

Make sure petcock vac hose is connected (and not leaking either in hose or at petcock itself). I'd disconnect it and plug #2 carb vac fitting to test things- run on "PR" spot. This lets you rule out petcock as a possible problem.

I forgot to mention, I installed new petcock too. I haven't checked electricals. Could there be low voltage at coils, or some sort of timing problem causing this?

[jsandidge]

leaks

Thanks for the replies. I've ruled out air or exhaust leaks. Any other suggestions?

[tom203]

..... or some sort of timing problem causing this?

Timing should be fully advanced by about 4k rpm- make sure the advancer (if you have one) ain't stuck. But your problem sounds carb related - if you're just reving it in neutral (not much load), try backing idle/air mixture screws out a tad.

[jsandidge]

screws

Timing should be fully advanced by about 4k rpm- make sure the advancer (if you have one) ain't stuck. But your problem sounds carb related - if you're just reving it in neutral (not much load), try backing idle/air mixture screws out a tad.

I'll give that a try. I'm a newb. I thought those screws only affected the mixture at idle. I set them at 2-1/2 turns out after my carb clean and haven't messed with them since the bike idles fine.