Topic: it RUNS! but ....

[74cb750]

Ok. The 1974 cb750 now has rebuilt carbs, synced them and it backfires at 55-60 mph. Damn, they are such a pain to get in/out that I decided to put K&N filters on. Any ideas ? michel

[Gordon]

Did you re-jet the carbs to compensate for the pod filters?

[Tim2005]

Backfires? You mean like loud bangs out of the exhaust? IMO that's unlikely to be a carb problem, more likely to be caused by an ignition fault such as a bad connection, causing the ignition to cut out momentarily then back in again, or the header gaskets sucking in air, which is usually spotted if the backfires are when you are on the overrun.


Cheers

[74cb750]

The backfires are small, like when cruising at 50mph, small backfires. And lack of power on hills, almost like my 2nd bike, a GT250. I'm doing a compression test this morning. Hopefully that will tell me something.

[Badboy]

I would say that you have an ignition problem.

[Caferacernoc]

I think you need to re jet for the pod filters.

[TwoTired]

I think you need to examine your plugs and compare them to the pics at:

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

If they are not all 4 similar in condition that should be noted and addressed.

[dpen]

Attack this in a lgical way, obvious & easiest first.
Spark plugs ok?
Points condition, gap & dwell ok?
Ignition timing ok?
Odds on it will be fixed by now, if it's not then you can start on the fuel system secure in the knowledge that the ignition system is ok.
Presto-you've narrowed the problem down.

[MetalHead550]

Did you sync them before or after the k&ns?  Anyway, as others have said, youve almost certainly created a lean situation by putting pod filters on without including bigger jets.  Your carbs are now able to breath better, but now they get more air without getting more gas for the correct fuel mixture=lean.  Yep, check your plugs to confirm this.  Good luck man.

[74cb750]

ok, dpen I'll try all that in order....spent the day working on a 1973 Z1 for a friend and never got around to mine..

[dpen]

Let us know how you got on. Depending on your result, could be a shortcut in there somewhere.

[74cb750]

Ahhhh well, it's like this....Went to Loudon, NH to do burnouts with the 74 cb750 and, blew a headgasket. Aha! So that was the problem. Drove it home anyways running pretty much on 3 cyclinders and on 4 every once in a while. Now, I either finish rebuilding the 1978 motor I have, tear down this 1974 or put gas in the 1975. Hmmmm what should I do? Think I'll put gas/spark plugs/etc in the 1975 and take it off the market for the rest of the summer. I can always rebuild the 78 and the 74 over the long cold, dark, depressing, etc winter, non? PS If the guy whom never showed up yesterday to buy the 1975, finally does show up what would you suggest I tell him? And what color should I paint my 1975? see pictures of MR. Ugly at http://www185.pair.com/canukvt/hon/1975_cb750/  It is no longer for sale.