1971 Honda cb750 idles & runs rough up to 3-3,500 rpm

gwminor48 · L Plate Warrior Joined: 09 Sep 2005 Karma :

This bike seems to have the electronic type points. It idles & runs rough until about 3,500 rpm & then it runs great. It makes it tough starting from a stopped position. Does this sound like a timing problem? It also makes a noise at idle which they thought was the timing chain where I bought it. I tried adjusting the tension on it it per the Honda & Clymer methods but it hasn't seemed to help. I was trying to adjust the timing from the points when it died and the old battery wouldn't restart so I need to replace the battery. I may be in for more work than I thought. It has 42,00 miles (if that is the correct reading), I bought it a month ago.

Thanks,
George

mr.z · World Chat Champion Joined: 04 Feb 2004 Karma :

Try changeing your air filter and spark plugs, both these could cause crappy idleing, otherwise (this one is more likely but more hassle hence try it second) its the carbs, if its been stood for a while they might need cleaning, take them off and give then a good clean out (idle jets especially).

Good luck Wink

(cool bike by the way Thumbs Up

)
____________________
>RidingSkills<->Tech Tips<->MyBikes<

map · Mr Calendar Joined: 14 Jun 2004 Karma :

An idea would be to check/renew your HT leads and plug caps. I'm thinking if it's a problem at low revs (therefore low alternator output) if the leads are breaking down this will cause a problem for the current. At higher revs you get higher current and therefore it can 'jump' over any breaks.

Always a good idea to do a mini service (oil, filters, plugs) on a new to you bike as then you know it's done and can arrange for it to be redone from that date/mileage. Also a good idea to replace leads every 2-3 years (although that may be me being cautious). If you don't know when the last time this was done it might be an idea to include with the service.

Just a few thoughs Thumbs Up

____________________
...and the whirlwind is in the thorn trees, it's hard for thee to kick against the pricks...
Gibbs, what did Duckie look like when he was younger? Very Happy

Bezzer · World Chat Champion Joined: 14 Apr 2005 Karma :

I'm with Mr Z,clean the carb, it's the first and cheapest option, as in free, could well be the idle circuit part of the carb then it picks up when it comes onto the main.
Does the noise go when you pull the clutch in? if so just clutch rattle, no great problem.
____________________
I used to be indecisive but now I'm not quite so sure.

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

gwminor48 · L Plate Warrior Joined: 09 Sep 2005 Karma :

I really, really appreciate your replies, I think you are all onto something. I got the bike started again and fooled around with the timing and it was running (not idling) really good for awhile (10 minutes. It got worse but not nearly as bad as yesterday. The difference is it was 103 degrees yesterday (I live around Dallas, Tx) and 73 degrees today. When I got the bike the plug wires were in terrible shape and it seems you have to replace the coils too unless you are really inventive, so I bought coils with wires in pretty good shape from a wrecking yard type place. I also replaced the plugs and changed the oil and filter and clean the fuel screen in the petcock. I haven't changed the air filter. The container for the filter says cb750f (which it is not), so I'm not sure about the filter. The points are after market, what looks like the points are entirely covered in rubber. So I'm thinking that since it worsens once it warms up maybe it is in the carbs. Also, once I adjusted the cam chain per Clymer with the cam in the right place the noise seems to have quieted down. Thanks so much for your help guys!