Poor High Speed Performance

[sdsguru]

Newbie post I read through several posts, but didn't seem to find anything that deals with my specific problem.

The Bike: 1992 Honda CB750 (Nighthawk). 15k miles, in pretty good shape. I just bought it a week ago.

The Problem: I took my first highway ride this morning, and I was having very poor high speed performance. The bike could just barely hit 70-75, but I was loosing speed going uphill (even on slight grades). I could maintain 55-60 MPH, but anything above that and the bike just didn't have any power.

Background: I think the bike has been sitting for some time. I suspect carb issues, but a friend also suggested drive-train/clutch issues.

I filled the tank with super (93 octane) gas, plus I put in a ¼ bottle of octane boost. Yesterday, I took the air filter out and shot some spray-type carb cleaner into the air intakes. After the bike quit smoking it seemed to run a little better.

The bike is a little hard to start. I usually have to hit the throttle to get it to actually start running, and it runs a little rough until it's warm (I'm told this is normal). I've noticed a little bit of backfiring when I decelerate. I've also noticed just a little bit of surging while I was riding around on side roads.

The solution: If I can solve this one without (1) having to put it in the shop or (2) getting a kit and rebuilding the carbs myself I would really like that (wishful thinking, eh?) I'm a fairly competent wrench turner, but I'm new to working on bikes. Plus I don't have any of the guages to do carb work. I have a shop manual for the bike as well. Not afraid to pull the transmission cover off either.

Any suggestions? Is this a problem that may work itself out after a couple of tanks of high-octane? I should probably clean the fuel strainer and maybe thrown in some new plugs. Could this be drive-train issues?

Thanks!

Stu

[poppygene]

Probably needs a fuel system cleaning, Stu. Drive-train? Not likely, based on what you said. However, if the RPMs are rising without an increase in MPH, well... that's a different story.

What won't help: High octane fuel nor octane boost. Your Honda should do fine with 87, and 93 ain't gonna clean the crud out.

What may help: SeaFoam (from the auto parts store) may clean things out fairly well. I've had good success with it in situations like this, but it may take several doses to see improvement. Standard dosage is one ounce per gallon of gas. New spark plugs won't hurt but you may not notice any difference.

What would be better: An all-out attack on the carbs, including disassembly, soaking in carb dip, re-build kits and a good synchronization. BTW, a decent carb-sync kit can be had for around $40.

I'm glad you have a manual - lots of guys try to work on their bikes without 'em.

Oh... welcome to the forum!!!

[sdsguru]

Thanks. I may just have to tear the carbs down and clean 'em anyway.

I'll try SeaFoam and a few other "external" things and see if that helps.

[sdsguru]

Update: After two tank's with Seafoam, my bike is running much better! poppygene, you're a genius!

I still need to clean the carbs, but this will get me by for a few more weeks.

Thanks again!