Honda Motorcycles - FireBlades.org banner

1 - 14 of 14 Posts

·
Registered
Joined
·
3 Posts
Discussion Starter #1
Hey Guys,

I am new here and I am in need of your expert advice! I just purchased a 1991 Nighthawk 750 that was not running, but I was told it used to run fine, but has been sitting for awhile.

So far we have changed the plugs, the battery, the petcock, the air filter, the oil, and degreased the engine. As of last night the bike will start and run at full choke, however as soon as we either lower the choke (almost off) or give it any gas from the throttle it dies.

It does not sound like it is getting too much gas, I am thinking too much air. It is also smoking a bit from under the bike. Is this a carb issue or possibly something else I should look into? We removed one carb cap and sprayed some starter fluid in. The bike would throttle fine until the fluid would burn off and then back to dying again.

I don't want to rebuild the carbs if there is something easier I can try first. Any help would be greatly appreciated.

Thanks,
Jeff
 

·
Registered
Joined
·
3 Posts
Discussion Starter #3
Yes, fuel was changed, we even ran some premium through, hoping to burn some of that stuff out. So what would you recommend for cleaning the carbs? Or should I be looking at getting a couple sets of carb kits?
 

·
Registered
Joined
·
900 Posts
If it was down to me i would strip the carbs down and clean out the jets, if you want her to be reliable, just use any carb cleaner, you will need to clean plugs again. when you do strip it down check all rubber seals for splits and vacuume pipes. Oh undo the tap from the tank most have a filter there, it might be clogged up, worth a look.............:thumb:
 

·
Registered
Joined
·
2 Posts
I have a 96 with similar problem, I cleaned the carbs - they honestly didn't look bad at all. Had very little carmel in the residual carb cleaner. Put them back on and it does run better but still have to have the choke on halfway to get it to go to 8k rpms No choke and it will not go past 5k rpms. When the choke is not on and ramp up slowly to 5k it will go to 5k and then eventually sounds like it runs out of fuel.

Oh, I must of not have put the throttle cables on exactly correct because the throttle does not return to idle on its own anymore. Is there a quick fix for that. The cable are not reversed because the rpms increase and decrease properly to the direction of the throttle pull or return.

Any help is appreciated.
 

·
Premium Member
Joined
·
32,034 Posts
I have a 96 with similar problem, I cleaned the carbs - they honestly didn't look bad at all. Had very little carmel in the residual carb cleaner. Put them back on and it does run better but still have to have the choke on halfway to get it to go to 8k rpms No choke and it will not go past 5k rpms. When the choke is not on and ramp up slowly to 5k it will go to 5k and then eventually sounds like it runs out of fuel.

Oh, I must of not have put the throttle cables on exactly correct because the throttle does not return to idle on its own anymore. Is there a quick fix for that. The cable are not reversed because the rpms increase and decrease properly to the direction of the throttle pull or return.

Any help is appreciated.
Carmel?
Sounds like it's very lean. Are all the header pipes similar temperatures?
Any air leaks around the intake manifolds?
Have you tried loosening the cables?
Get a workshop manual.
 

·
Registered
Joined
·
2 Posts
Thanks for helping.

By Caramel, I meant the brown looking build up that gets in the carbs. Very little.

Any idea how to change the mixture on the 750 Nighthawk? It would need to be done for all 4 carbs at once I would think. What the best way to check for air leaks?


I will try loosening the cables for the throttle issue - that would make sense
 

·
Premium Member
Joined
·
32,034 Posts
Thanks for helping.

By Caramel, I meant the brown looking build up that gets in the carbs. Very little.

Any idea how to change the mixture on the 750 Nighthawk? It would need to be done for all 4 carbs at once I would think. What the best way to check for air leaks?


I will try loosening the cables for the throttle issue - that would make sense
The mixture screws are in the bottom of each carb but they shouldn't need altering once they're set unless something has changed.
For air leaks, start the engine and spray WD40 around each intake manifold listening for the change in engine note as it sucks fluid through any gaps.
What do you mean about pulling one carb cap to spray starter fluid in - do you mean you pulled one of the vacuum slides out?
 

·
Registered
Joined
·
2 Posts
Hello Bladeracer

How are you?

I see that this tread is from 2010, but I take my chances that you will answer.

The other winter I rebuild my 96 Nighthawk to somekind of a start of Cafe racer. Everything that had with air, before the carburators was removed.
Now there is separate air "plug" filters, one each for every carburator.

Do not have much problem when starting it. But when engine is warm, I have to work it up, slowly slowly to 2200 rpm after that I can put some more gas to it.
This was not the case before.

I just assume that is the gas/air mixture in the carburators, I´m just starting to investigate.

Do you have any good suggestions and advice how to "fix" the carburators? and how to test it the best way, with out needing to get the carbs on and of, 20 times.

Thank you.

/peter
 

·
Registered
Joined
·
1,075 Posts
I clicked on this thread because my wife has a 1992. I don't look forward to needing to work on it.
 

·
President: Team Full Chat
Joined
·
8,154 Posts
Hello Bladeracer

How are you?

I see that this tread is from 2010, but I take my chances that you will answer.

The other winter I rebuild my 96 Nighthawk to somekind of a start of Cafe racer. Everything that had with air, before the carburators was removed.
Now there is separate air "plug" filters, one each for every carburator.

Do not have much problem when starting it. But when engine is warm, I have to work it up, slowly slowly to 2200 rpm after that I can put some more gas to it.
This was not the case before.

I just assume that is the gas/air mixture in the carburators, I´m just starting to investigate.

Do you have any good suggestions and advice how to "fix" the carburators? and how to test it the best way, with out needing to get the carbs on and of, 20 times.

Thank you.

/peter
Hi Peter, don't expect a reply from BR, he's on a 4 year (so far) hiatus.

The Nighthawk is an extremely simple bike, so I'll start with some easy questions.

You have individual "pod" filters for each carb, correct? Please post a picture of the set up.

Aside from removing the entire air box assembly, what other modifications are different from before you started?

You may need to adjust the fuel/air screws, but generally that circuit will only affect idle and start up.

See if your fuel line is kinked, or if it has a vacuum operated fuel valve, that is it pulling vacuum properly.

Is your choke lever/cable set up and working properly? It's an air cooled engine, it may take longer to get to normal operating temperature.

Make sure your fuel is not from last year, if so, remove and replace. And make sure it's not Diesel fuel, I've seen it happen before.



I clicked on this thread because my wife has a 1992. I don't look forward to needing to work on it.
Jay, it shouldn't give you any trouble, if it does, as I said above, it's an extremely overly simple machine. You should be OK.
 

·
Registered
Joined
·
1,075 Posts
Thanks for having confidence in me @IanDoohan. I'm just recently getting into carburetors, considering that we have the Nighthawk and now 3 dirtbikes. Everything else I've ever owned was FI, including my first car - a 1983 model.
 

·
Registered
Joined
·
2 Posts
Hello Ian

Sorry late answer.

I looked through the whole line and understood that I have a leak and as I understand the black little "ring" inside the fuel filter was not right and clogged the fuel feed.

My NightHAwk is in a middle of a re-building, so I´m going to adjust the carbs, with new jets and so, and I´m going to renovate the Fuel Switch, after that I can see again what happens.

Thank you very much for you quick answer ... Do you have any thought or tips about what I should NOT forget.
This is my Project Number 1 when it comes to bikes, so Im a totally fresh man, but I´m looking forward to make this Cafe.

Have a good night.
 
1 - 14 of 14 Posts
Top