Author Topic: backfires on startup (Read 6115 times)

Heirborn · « **on:** November 19, 2005, 09:17:08 AM »

My '70 cb750 backfires after I start it. Turn the choke on, have to hit it a few times before it runs, I adjust the choke down, twist the throttle to keep the R's up, it bogs a bit, backfires, (left side only so it it either out of 1 or 2) it does this 2 to 4 times and then runs fine. What do I need to look for?

Heirborn · « **Reply #1 on:** November 19, 2005, 09:49:32 AM »

only if I want to look like every other rider on the road.....and that's not my bag baby!

TwoTired · « **Reply #2 on:** November 19, 2005, 10:08:12 AM »

Quote from: Heirborn on November 19, 2005, 09:17:08 AM

My '70 cb750 backfires after I start it. Turn the choke on, have to hit it a few times before it runs, I adjust the choke down, twist the throttle to keep the R's up, it bogs a bit, backfires, (left side only so it it either out of 1 or 2) it does this 2 to 4 times and then runs fine. What do I need to look for?

What is the tuneup history; itemized list?

Is it in stock configuration? What's different about it?

Had the carbs synced?

Any exhaust leaks? port to tip?

Any recent carb work done?

ProTeal55 · « **Reply #3 on:** November 19, 2005, 10:38:58 AM »

My CB backfires outta the carbs after it is started , and as it warms up.
The colder it is , the more it backfires.
I Think it is a jetting issue.

As soon as the bike is warm , goes away, and runs fine.
I am not worried about it , gives the bike it's charm......

Jonesy · « **Reply #4 on:** November 19, 2005, 11:07:41 AM »

My 750 does it too, mainly in cold weather. One or two pipes will pop a few times until it's warmed up. It's like a couple cylinders take a minute to wake up. Then it runs great.

eldar · « **Reply #5 on:** November 19, 2005, 11:55:09 AM »

Air leak could be as good possibility. Otherwise a rich mixture or poor spark can do this.

Heirborn · « **Reply #6 on:** November 19, 2005, 04:03:19 PM »

I bought the bike 3 weeks ago. Bike was hitting on cyl 2 on and off. Found out the battery was bad. put a new battery in it, clipped the spark plug wires, reinstalled the caps, adjusted the valves and adjusted the points, changed the oil and filter, have not checked the timing. Got the bike running and is hitting on all cyls. Idles fine. I do have to blip the throttle from time to time to bring it back to idle. A little bit of black smoke out of the left side when the throttle is gunned, not too much, but enough to see which should mean a little too much fuel. Bike seems to run ok....don't have much to gauge it by. Last time I rode before getting this bike was in '84 on a new Interceptor.

I think that is it....all I can think of now.....

I bought the bike knowing that it is a project and enjoy messin' with it. I plan on bringing it back to original as possible. I don't plan on it being a show bike but a good looking rider.

Thanks for everyones help.

TwoTired · « **Reply #7 on:** November 19, 2005, 08:56:41 PM »

Good work so far, but you haven't completed the tuneup yet. Before playing with the carbs, you still have the timing to do, the Cam chain tension to adjust, the air filter to clean/replace, the spark plugs to clean, gap, verify correct numbers.

Then you can check pilot screw settings, carb sync.

Then you check head pipe temps at idle for even temps. A cold one may be blocked slow jet.

If you have changes to exhaust sytem or intake from stock, you may have some carb jetting/tuning to do.

If you still have backfire issues at this point, check for induction leaks or leaks in the exhaust system.

If you run out of ideas, give us an update and ask more questions.
Cheers,

Heirborn · « **Reply #8 on:** November 20, 2005, 05:27:53 AM »

Thanks. I did fail to mention that I did adjust the cam chain tension and all the pipes are hot, air filter is clean. I will check the plugs and set the timing today and see how that helps out.

KB02 · « **Reply #9 on:** November 20, 2005, 06:33:46 AM »

I would say replace the plugs, too, instead of just cleaning them. As inexensive as they are, if you just got the bike, it wouldn't be a bad idea to start out with fresh plugs.

TwoTired · « **Reply #10 on:** November 20, 2005, 10:58:00 AM »

Just a caution about the air filter. Used paper air filters can look clean but actually be restricted with particulates trapped during the filtering process.
Honda guidelines are to clean at 6000 miles and replace at 12000 miles for that reason. Of course, dusty conditions require shorter intervals.

If you don't know how old the plugs are, they have a 6000 mile cleaning interval and a replace when neccessary recommendation. My own experience is one, maybe two, cleanings with a bead blast and electrode reshaping with a file (to restore sharp edges), is about the limit before replacement.

Cheers,

ProTeal55 · « **Reply #11 on:** November 20, 2005, 02:22:57 PM »

I put a fresh set of plugs in all my toys when I revive them from their winter nap.
Worth the money in my experience...

Heirborn · « **Reply #12 on:** November 22, 2005, 06:13:12 PM »

Repalced the plugs, all were pretty sooty....found out they were one cooler plugs then "recommended". Put the right ones in and it did not back fire. The butt dyno indicates more power. I did not get to check the timing. After it has heated up, gets a funky idle. Idle high, then drops then runs back up again. Gotta love a project!

ProTeal55 · « **Reply #13 on:** November 23, 2005, 01:14:11 PM »

The changing idle sounds like vaccum leaks to me.
While the bike is running , spray some break cleaner around the rubber
boots , and see if the idle changes. If it does , you have airleaks...

Bodain · « **Reply #14 on:** November 24, 2005, 05:24:38 AM »

My CB 550 does this also.. It only seems to happen while the engine is warming up. Quits after the engine is warm.
I looked for intake and exhaust leaks. I don't beleive it's a carb issue. I realy don't know why it does it. The symptom
seems to be very common for older CB's as seen in this forum. I don't concern myself with it anymore.

Jonesy · « **Reply #15 on:** November 24, 2005, 05:35:20 AM »

Ditto here. Everything has been rebuilt on mine, new carb rubbers, band clamps... the works. Pops and bangs a bit on warm-up, only in cold weather. I just accept it for what it is..

dusterdude · « **Reply #16 on:** November 25, 2005, 07:15:22 AM »

it may be a wee bit lean,mine does it too.

Briboy · « **Reply #17 on:** December 01, 2005, 11:31:33 AM »

Hi Heirborn
My CB750 K6 has the same problem on start up,I have to keep throttle open till it ticks over but I have two pipes getting hot and two staying cold,been told its the carbs getting it looked at tomorrow let you know how i get on.

ElCheapo · « **Reply #18 on:** December 02, 2005, 05:26:28 PM »

According to Mikuni Lean back fire is through the carbs and rich is out the exhaust. Teal, This explains why yours quits behaving badly after it warms up. Maybe a sixe bigger on the jets might not hurt, What color are your pipes?

Briboy · « **Reply #19 on:** December 08, 2005, 11:47:56 AM »

Hi Heirborn
problem i had when starting from cold has now been sorted,no more backfires and all four pipes getting hot,the problem was dirty carbs and they needed setting up,and the old petrol didn't help.

News:

Author Topic: backfires on startup (Read 6115 times)

eldar

Briboy

ElCheapo

Briboy