Slight popping noise backing down in gear

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

Is there any way to get the popping noise out when the motor backs down in a gear? Its got the 78 std factory double wall pipes with Dunstall Replica mufflers on it with a thin shim in there to take up the difference in space, I think its sucking air at the joint but doesnt leak to hear it. Ive been thru the carbs twice and they are spotless and the bike runs & idles perfect except for that. I can live with it but Id rather not. All jetting is stock 78 specs w/factory air box & filters

jayel

butter those shims up with some hi-temp rtv silicon to eliminate any leak at the joint, if it doing only on one side you're still a little lean on that side open the air mix screw 1/8 to a 1/4 turn and see what it does

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

Ill try it, they are about 2 1/2 out now, set at highest idle speed. I was thinking of packing the space between the inner & outer pipe with something. Dont know if it would help or not.

jayel

I use that copper colored hi-temp RTV and no leaks, there will always be some variation in carb settings, just one of the problems with running two carbs

gggGary · Full Member Joined: Jan 07, 2009 Posts: 114

have you synced the carbs with gauges? a slight variation in throttle plate opening can cause that also.

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

Yes, I did sync them with mercury sticks. Adjusted the carbs 1/4 turn out but wasnt able to try it today. Thanks for al the advise guys, this is a great forum ! Found the right spring for the idle adjustment screw I lost a month ago and replaced the cut down ball point pen spring I had in there. That might help too, thats the cyl that pops

pamcopete

Well, I found the best way to match an aftermarket muffler to the double walled headers is with the use of an adapter:

Part number 548520, available at most auto supply stores. It's a 1 1/2 inside diameter to 1 3/8 outside diameter.

I used it to install my mufflers on my '81/H stock double walled pipes.

It looks like the muffler is clamped to the outer header pipe, but not so. It is just butted up against the outer pipe.
The smaller diameter of the adapter fits over the inner pipe. The mufflet fits over the larger diameter. It's such a tight fit on the inner pipe that I didn't have to use any sealant, but if you want to, you can use a muffler sealant.

I think a lot of people just clamp the muffler over the outer pipe which is not designed for any pressure and in fact leaks at the header flange and has two drain holes.

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

Theres no drain holes on a 78 std that I can find but Ill have one of them today & try it.

pamcopete

woodman

The other thing you can try is to adjust the sync screw to get both exhausts to sound the same at idle. No more than an 1/8 turn of the screw either way until they both sound the same. Works best if both mixture screws are set the same number of turns.

I do this on my '81/H as a final tune up when syncing. I have both carbs set to factory spec on the mixture screws (3 turns out) and the jets are factory.

Finish up by connecting the vacuum barbs together with a "T" or two for the pet cocks.

woodman · Full Member Joined: Oct 12, 2008 Posts: 256

I got the exhaust adaptors but the 78 std does not have the space between the pipes like a Special does, they are double but tight up against each other. I cut the small end down and slotted it & drove it up inside then put the mufflers on. Unfortunately after all that it does the same thing. I looked in the Dunstall replicas and they just have a grate inside and the holes on the end, no glass packing so maybe thats just the way its going to be. Same deal with the carbs, nothing makes any difference, Ill just live with it until I can get different pipes. Thanks for the help anyway

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