Backfiring Nighthawk

[KingGroover]

What's up, maintenance studs.

Can someone give me some advice re: a poor power/backfiring problem that I have on my CB750 Nighthawk?

Engine idles well, but when I give the throttle a good twist from idle it bogs at 2-3000rpm. It then continues to climb to the upper rpms with no apparent problem. When I close the throttle sharply, I get backfiring.

I had the carbs serviced, nice and clean now but no change to the bogging/backfiring.

Same behavior both with fuel tank/strainer and external fuel cell/no strainer.

Changed plugs, same behavior. Plugs generally look good but the white ceramic insulator gets a slight brownish look to it when I make the bike bog and then remove.

Coil primary resistance in spec for both coils.

Secondary resistance (with caps) slightly high for both coils...spec is 18-22kohms, I get around 24 and 25kohms on the two coils. Secondary resistance without caps is in spec.

Battery voltage is 11.85V. Battery's been in there awhile and I haven't ridden it regularly, but no problems starting etc

Any ideas? :?:

Thanks

Rob

[Northman]

Sounds lean to me.

Try running it with partial choke, and note any improvement. If it improves, then you're definitely lean, and the problem is carb related. Having them overhauled should cure the problem, but check all vacuum line routing first to make sure everything is hooked up properly.

[aptyp]

Before you do ANYTHING else. Take off the rubber boots that go from the carbs to the engine and silicone those suckers to the engine so there are no air leaks. I've had this exact problem and that's what took care of it.

[xxxbirdxxx]

Before you do ANYTHING else. Take off the rubber boots that go from the carbs to the engine and silicone those suckers to the engine so there are no air leaks. I've had this exact problem and that's what took care of it.

If those boots are the problem just replace them! Check them for cracks or holes. You should not have to silicone anything that has to do with your carbs or intake!

Do as North says and see if your carbs are to lean.

Good luck and let us know what you come up with.

[Hobicus]

You can verify the whether the intake boots are leaking by spraying a little carb cleaner around them at idle. If the idle picks up, you've got a leak. If your CB 750 was anything like my 650, there's the intake boots, which are rubber, and a plastic like spacer with 2 O rings, one on the boot, one on the spacer where it attaches to the manifold. I had to replace the both O rings in all 4 intakes to fix my leaky intake problem. Once I did that, though... All running problems were solved.

You should also check the following things:

1. Make sure the boots on the AIRBOX are seated all the way, too. Use the carb cleaner trick to see if you have any leaks. It's a BITCH to get that airbox all the way in, but if it's not, it will cause a lean condition, and hesitation at midrange.

2. Make sure you don't have any exhaust leaks, as extra air being sucked into the exhaust can cause deceleration backfiring. My favorite way to check that is when the engine and exhaust are cold, first thing in the morning, you can usually see it when it's idling on choke.

3. IF your carbs have a pump diaphram, it may need replaced. It's a little diaphram that opens when the throttle is closed to richen the mixture to stop backfiring. I know my CM400 had them, but I dont' think my nighthawk had them. Mine was an 83, though.

4. Edited to add: Check your float bowl height, too, as having the floats too low will cause a lean condition.

If all that doesn't fix it, pull the carbs again, spray them out, and BLOW THEM OUT with compressed air. Reassemble and check all the stuff above.

Good luck, keep us posted!

[KingGroover]

It was lean.

I rechecked my hoses, I had the #2 cylinder intake to fuel tap taped off while using the external fuel cell, but apparently not well enough and it was still leaking air :oops: I'm a dumbass

Pulled the tape off and it was gulping air and backfiring. Sealed it up properly and it was smooth.

Checked the carb-intake boots for clamp tightness, then sprayed carb cleaner and no change in idle...good to see those were good...would have been a total pain in the ass to detach/reattach the airbox boots :mad:

I think I also had the idle a bit low, stepping that up seems to help the response.

Put it back together and took her out for a spin...runs fine albeit a bit slower than I remember (been riding the BB exclusively the past few wks) :wink:

Really nice bike, the Nighthawk...good looks and bomber reliable. Probably gonna sell it soon, shame to see it go.

Thanks for the help.

PS Nice pix, Rich :razz: