1978 GS750 Jetting

[Anonymous]

Hi all,

I just bought a 1978 GS750E with a 4 into 1 Mac pipe, and was wondering about jetting. I plan on keeping the stock airbox, and MAYBE adding a K&N drop in, but not individual pods... I am just looking for a good mixture, which will allow the bike to pull smoothly all the way to redline... I was hoping for a good needle/main jet combination to use. Please no pure race settings, as this is my daily-rider... Thank you all very much!!!

[Jeff Tate]

Install the new pipe and leave in the stock filter and see how she runs. I kind of doubt if you'd have to change any carb settings. Now if you add a K&N drop in air filter, you MIGHT need to maybe raise the needles relative to the slides (rich) and possibly go up one size on the main jets. Only trying one change at a time and testing it will tell you what it needs, but I'd be willing to bet this would about be the extent of carb mods, if any. Good Luck.

[Anonymous]

From all the comments that i have read on this site, the mack pipe is not a performance pipe; but one for dressing up a bike and therefore does not need re jetting--im just repeating what i seem to remember reading in some earlier discussions.

[Anonymous]

Should have been more specific... the bike already has a Mac, and is running a bit poorly, and I'm SURE it's lean, because years of sportbike experience tells me that the bike was running lean from the get-go, due to emissions, and now the Mac pipe has introduced more airflow through the engine, so the carbs need to compensate with more fuel. I'm sure I won't need to get new needles, as the taper will most likely be correct, and I don't want to shift the powerband with a different taper, but I do need a new needle jet size, and possibly a new jet needle too. Any past experience which has led to good, proven results would be most helpful.

[Anonymous]

what color are your spark plugs please

[Anonymous]

I wanted to address a couple of things in this email, the first being the request for a spark plug check... If a perfect tan were a 5 and a white "too lean" condition were a 1, then I have a 3-4. I did a correct plug test, which is as follows: the bike is burbling and hesitating a bit from 4500 rpm to 6500 rpm, then it picks up and pulls cleanly to redline. It also pulls cleanly from low revs. I did the check on the way home, running the last mile or so at approximately 4500 rpms, then turning the kill switch to off, and coasting into the driveway. The reason is because I wanted to check to see what the plugs were doing at the rpm that I was having problems. I like the idea of checking plugs, but it's not accurate for every part of the rpm range. For instance, if you had excessive wear on your needles, you could be running perfect at one rpm, and badly at another (this information is more for the newer people to engines). I do plan on raising the needles this weekend one notch, but I would like some accurate jet sizes. In response to the Mac is only a show pipe, I do not think this theory is accurate, because for one, the resonating chamber (or canister) was changed to sound better, which affects airflow, and for another the Mac system is a four-into-one, which dramatically changes the affects of airflow from the stock exhaust. I think that's all for now.... Thanks for the response so far, I enjoy a good discussion!

[Jeff Tate]

Hi again. Try this website for alot of good, specific, printable information on jetting procedures for mikuni cv carbs. http://www.factorypro.com/tech/carbtun.html

[earlfor]

For a 79 750E (same carbs as a 78 model)with a vance and Hines 4 into 1 header and the stock airbox, you would want to change jets to 17.5 pilot and a 115 main in mikuni jet sizes. I suspect the same sizes would work acceptably well for a mac header.

Earl

[Anonymous]

Might want to try synching the carbs first

-B