78 carburetor questions.

[Don R]

I did the auxillary jet install in the hoses going to the pilot air jets, easy, quick and appears to have made a difference. The bike was in the garage and started out warmer than outside air but it ran slightly better and the lower RPM's air/fuel ratio looks better on the wide band gauge. The new air filter looks exactly like the old one, I may have ordered one from Randakk when I first got this bike. Too many bikes to not keep log books. The amazing grips were nice too. I didn't do the bowl gaskets yet.

[jimbo]

If I was having all those carb issues I would swallow my pride and send them out to the professionals that do this all the time and have an engine that they can bolt them on to make sure everything is working correctly, I did am I’m so happy not messing with them anymore, I sent mine to robin ray and there perfect

[redglbx]

One thing I would suggest is changing the “air” jets in the brass elbows. Your stock 78 air jets are #120’s ,I would replace them with #110’s, quite easy to do and will improve the low speed leaness. They can be had from several different suppliers.

On several of my bikes I found a jetting combo while on a dyno with a lambda exhaust sensor that really works well. In addition to the smaller #110 air jet I also changed the low speed fuel jet from a stock 62 to a 65 (1975)and then reduced the secondary mains from a #120 to a #115 as the stock low speed jetting is really to lean but helps keep the motor clean and free from loading up & fouling the plugs.

But the high speed jetting is way too rich with the #120’s and the cumulative fuel from all the jetting. So a reduction to the 115’s makes it much better. Anyway that’s what I do to help balance the fuel curve.

[rcmatt007]

Interesting in that my 78 has tended to ping on acceleration except when I run premium

[redglbx]

Roger, that’s interesting as my friend who bought a new 78 back then also had issues with it pinging. He took it back a couple of times to the dealer to fix it but they never did. His tended to do it in the lower rpm ranges if I remember correctly. Running premium tended to prevent it as well.

I wish I had known then what I know now as I would do a few things to improve that. I do believe that running the #65 low speed main fuel vs the oe #62 and running the smaller #110 air jet rather than the stock 120 would have maybe fixed all that.

BUT,,,

It kinda depends , if it’s the 78 “emission” carbs (so you need to check the carb number) if it is the “emission “ carbs I’m not sure of whether or not just some jet changes can fix that. I did a set of carbs for a friend a few years ago and the oe owner always complained about how poorly it ran and as I got into it, it had 2 oe LTD carbs, 1 77 carb and 1 of the 78 emissions carb that was very different from the othe 3.

Personally I wouldn’t use any of the 78 emissions carbs even in a 100% correct restoration unless it was going to be a static display piece. From what I could see of the 1 that I had it was a very poor example of trying to be emissions compliant. Now maybe some of the other guys here may have had these nd had good luck with them but I could see a lot of work in them trying to get them to work well.

[rcmatt007]

yes, they are the later carbs. As I bought it in August (12th) of 1978. And it pings exactly as your friends bike did. I guess the one positive is just toodling (like driving around in the Ozarks a few years back) it gets over 50 MPG.

I do believe I have a set of earlier 78 carbs, but I have not taken them apart wo I do not know what condition they are in. As for my 76 Ltd (still having flooding issues) God only knows what was done to the carbs from a PO as the floats were so off, and as it least one screw for the bowls was an "oversize) as it had been stripped

[gltriker]

snip-
As for my 76 Ltd (still having flooding issues) God only knows what was done to the carbs from a PO as the floats were so off, and as it least one screw for the bowls was an "oversize) as it had been stripped

Flooding issues... closely study the details in this Topic .

https://ngwclub.com/forum/viewtopic.php?t=78012

Watch My *Rack* Pressure Testing Video near the bottom of POST#10 too.

[rcmatt007]

yes, I know I need to pull them. Ran great after the original rebuild (okay with surging)... but I live 300 mile away 4 days a week, and mow the lawn every other 4 day weekend

[CYBORG]

Bought my 78 in 80. Second owner. Had 1000 miles on it. Completly stock. Never could keep the points correct for very long. Went to a Dyna unit. Better, but over the years went thru three units. 50MPG was not unusual on the hwy. Never had carb problems, although I have rebuilt them a number of times over years. When it was stock, never had any surging or back firering problems. Now run a C5 ignition. Vast improvment Stock carbs and jets. Half a million miles. ....Just saying

[rcmatt007]

yes.... dang until my carbs needing rebuildig, great bike... just a GREAT tourning bike

[redglbx]

Just an fyi,, you can get carb jets at PJ Motorsports who I use all the time orJets-Are-Us. No soldering or drilling jets just get the sizes you want.

The jet combination I have used on the 75-79 carbs that works very well to level the tailpipe lambda readings is:
Idle jet, Stock (on all Kehin carbs) 35 jet
Primary Main, #65
Secondary Main, #115 (#120 on all except the 75’s which is to rich)
Air jet in the elbows, #110

I have used this combo on numerous bikes and it works well, no off idle stumbles, better overall power with improved fuel economy. Just seems to work well.