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1983 Honda Magna 750 with Carberator problem

All the carbs were professionally rebuilt a couple months ago. Old fuel was flushed out and an inline fuel filet was installed before restart.

Starts and idles fine but when you give it gas it dies.

Was wondering if there was not enough fuel pressure being supplied but don't know how to test that.

Any input is appreciated.

I did post a youtube video with the problem, if you go to youtube and search 1983 Honda Magna 750 Carb Problem it will be one of the first ones.

 

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Starts and idles fine but when you give it gas it dies.
Are you using the choke.....at or near max. until it warms up good ??
Does it have a stock air box and filters in place ??

Has it been that way ever since the carb work ??
If so, why have you not taken it BACK to that "professional" ????
 

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Discussion Starter #3
As far as warm up goes, it has the same problem when its hot I will let it sit and idle for 30 mins - still same problem. although I do not leave it at full choke except for startup.

It has a stock air box and filter.

It drove fine almost everyday for a couple weeks before this happened.
The professional is in California and I am in Florida.

I just dont know if this is a carb problem or not???
I am wondering if the fuel pump is not working right but I just don't know how to test the carbs without the fuel pump or the fuel pump with out the carbs.
 

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I just dont know if this is a carb problem or not???
Yes, the odds are great that it IS.....either a carb problem or you got a batch of really BAD gas.

The fuel pump on a carb'ed bike just helps fill the float bowls and if it fails, the symptoms are different than what you are having.

I'd say drain the float bowls first......and maybe the whole tank if this problem started right after filling the tank.

THEN a healthy dose of a good carb cleaner might fix you right up; might.

Where is Fla. are you? I've got a can of B12 in my garage.
 

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Are all four cylinders firing at idle? Maybe it's just me.
 

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Discussion Starter #6
I am in Tampa, Ugg not wanting to take it a part again, Such a pain to get a part and back together.

I will do it :(
 

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Are all four cylinders firing at idle? Maybe it's just me.
THAT is a good point indeed. :thumbsup:

I am in Tampa, Ugg not wanting to take it a part again, Such a pain to get a part and back together.

I will do it :(
NOTHING I suggested would require taking anything apart; should be able to drain the bowls just by opening a drain screw, I think.
 

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Just a thought....you definetly have a carb problem but could be as simple as fuel starvation. You mentioned placing a fuel filter inline. was it directional? Look for an arrow or inlet and outlet marked on filter, if it were me I would put a vacume gauge between tank and carbs anything more than 6 inches hg is to much.
 

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if it were me I would put a vacume gauge between tank and carbs anything more than 6 inches hg is to much.
I would think that anything more than ZERO would be too much.

There never should be Vacuum in a fuel line.....unless maybe you are measuring between the tank and a fuel pump, and even then it would indicate a partially blocked line or filter.
 

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Discussion Starter #11
Good thoughts let me do some testing today and see where it goes. I will try to drain those bowls before removing.

I am unclear on the vacuum issue but will check the direction of the fuel filter.

Is there a way to run the fuel supply to the carbs without going tank to fuel pump to filter???

That would help me dial in where the problem lies - if it runs fine on an alternative fuel source than supply is the problem and vice verse. but I don't know how to approach setting up that scenario to test it???
 

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Pull a spark plug and check the color after idling a bit. Plus, how does the exhaust smell? If it's too rich you'll know. It *almost* sounds flooded. (Could be wrong, starved for fuel can act similar if it's bad enough).

It did not "start right up," IMO. My 84 VF700 starts on like the 3rd or 4th *revolution* when the fuel enricher (NOT a choke, though it ends up doing a similar thing - richer fuel mixture) is engaged.

I agree it also sounds kinda like it's missing, my idle is definitely smoother. Try pulling ALL the plugs to get an idea on what's happening in the cylinders. If one (or more) are wet, then you found at least part of your problem. Checking for spark on those bikes is WAY easier than pulling and fiddling with the carbs.

So, I say start with plugs, verify you have spark on all 4 before you do anything else (since that's easiest). The fuel pump doesn't really supply much in the way of pressure (not like a fuel injection system). You CAN rig up a gravity feed setup and supply fuel that way. Honestly, if the carb bowls are full it should idle a solid minute (or two), heck mine will get me to the end of my street if I forget to turn the fuel valve back on... (Don't ask...)
 

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Discussion Starter #14
OK Update

It also has a flat spot at between 3-4 K RPMs.

I also pulled the factory fuel filter off and the inline one and ran it for 5 secs just to make sure I did not have a blockage. Did Not.

Also fired up the fuel pump and while i did not have a graduated measuring method it it spewed fuel out pretty good.

I also gravity fed the fuel with out the pump and it was fine.

I drained all 4 fuel bowls and it did give me some lattitude.
I am able to accelerate now albeit very sluggiish with out it dying provided that the choke is about 1/4 on.

So we may have solved part of a mutipart problem by draining the fuel bowls.
 

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Discussion Starter #15
I will try pulling the plugs today but it does smell rich to me, enough that it will make you cough.

Pull a spark plug and check the color after idling a bit. Plus, how does the exhaust smell? If it's too rich you'll know. It *almost* sounds flooded. (Could be wrong, starved for fuel can act similar if it's bad enough).

It did not "start right up," IMO. My 84 VF700 starts on like the 3rd or 4th *revolution* when the fuel enricher (NOT a choke, though it ends up doing a similar thing - richer fuel mixture) is engaged.

I agree it also sounds kinda like it's missing, my idle is definitely smoother. Try pulling ALL the plugs to get an idea on what's happening in the cylinders. If one (or more) are wet, then you found at least part of your problem. Checking for spark on those bikes is WAY easier than pulling and fiddling with the carbs.

So, I say start with plugs, verify you have spark on all 4 before you do anything else (since that's easiest). The fuel pump doesn't really supply much in the way of pressure (not like a fuel injection system). You CAN rig up a gravity feed setup and supply fuel that way. Honestly, if the carb bowls are full it should idle a solid minute (or two), heck mine will get me to the end of my street if I forget to turn the fuel valve back on... (Don't ask...)
 

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I will try pulling the plugs today but it does smell rich to me, enough that it will make you cough.
Indicates one of 2 things (or maybe some combo of both)

Improper Jetting (too big - mine was like that when I got it, I think my 700 had a swapped set of 750 carbs, the jetting was correct for a 750 but WAAY to much for a 700)

No spark on one or more cylinders (probably 2 - one coils's worth).
Currently I would lean towards not having any (or not good enough) spark in one or more cylinders. When my Magna was jetted wrong it would start easy, idle and seemed to run alright except it refused to accelerate past 5k rpm. As in rev-limiter like just would not do it. It would also bring your eyes to tears to stand behind it.

Check your plugs and make sure you have adequate spark (nice bright blue spark).
 

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Discussion Starter #17
Ah ha

Pulled the plugs. The 2 cyliders that run off one coil set look and smell fine. the 2 plugs that run off the other coil set are carbon foiled and smell like gas.

I think I need to change out that coil pack?
 

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I think I need to change out that coil pack?
Not necessarily.

Those two plugs fire in series......and either one of them being really bad can effect both......as can a bad plug wire or boot.

Do the easy/cheap stuff first, including checking the connections at or near the suspect coil.
 

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Discussion Starter #19
OK well this is still a puzzle, maybe back to carbs?

I went and bought an ignition tester that you put on the plug wire and measure the spark.

All tested well, all dropped off at the same point about 3 kV

???

Put new plugs in, runs the same stalls on acceleration.

Should I have tested the coils at Higher RPM - that test was at idle.

???
 
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