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Discussion Starter · #1 ·
First off I have learned that I am too vague so I will try to be as detailed as possible. My bike shows as being built 8/70 but registered as a 71 CB450. I had found that I had a cracked float on the left side carb. I have been having some trouble with the carbs but it was only sporatic. The problem is this. The right side exhaust is running pretty cold as well as getting some backfire through the exhaust. None through the carb. I static timed it per instructions here. I then checked the timing strobe style. The timing at 1,100 rpm is spot on on both sides. When I try to check it at 4,000 it is slightly retarded. I have the 14H carbs and am running a 130 main, and 38 pilot as well as low speed. Air screw is just over 3/4 out. Compression is 168 on the right cylinder and 161 on the left with the throttle open. Coils are new as are the points and condensor. I am also getting good spark on both cylinders. I really can't tell you if carbs are truly synced since I have an exhaust leak on the left pipe but intake pressure seems to be the same. So my question is what could cause the right cylinder gas to be coming out cold but seems to be running strong at idle. How to correct/what's causing backfire through right cylinder? do I need to adjust the timing for 4,000 rpm and if so then does that involve bending the tabs? Oh yeah gap is within specs. Should I maybe decrease the float height on the right side? (They are both at 20mm) I just cleaned out the carbs last week in case of rust and I am going to seal the tank next week. I also did the straw test and they both passed. Intake boots are in great shape with no visible cracks. Thanks for any help.
 

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Sensei
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A few possibilities right away.....
The "leaky" joint at the header/muffler will cause "backfiring" so fix that FIRST..... If that isn't the cure, then proceed as below:
Since the "nominal" (161PSI) cylinder is running the best, and you claim good spark, I would first TRY to adjust the timing for 4000ish RPMS. You would rotate the entire plate to do this, without changing the actual point adjustments.....I would generally look to get it on the more fully advanced line rather than centered in between the two lines.... If this solves the problem, Great!....If not, set it back to the EXACT firing at idle position.......
However, I'm more inclined to believe that it is a carb sync/fuel supply issue (if it's not the pipes), which could be checked by increasing the idle screw setting on the intermittant side and decreasing it on the "running" side to get a smooth continual firing on both sides at idle...... Low compression bikes often will need to be set up to 1200RPM to maintain idle once synched, and your one cylinder is below minimum specs.... (You could be overcompensating for the left's low compression and setting the idle higher on that side, and then having to reduce the right-side idle too low so it doesn't overrev, causing the sporadic firing on the right)
Let us knowthe results.......
 

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Discussion Starter · #3 ·
I will be fixing the leak as soon as I get a muffler to header clamp. I am thinking the same thing you are on the fuel supply issue. When you say increase and decrease the mixture screw I am assuming that you mean increase = richen and decrease=lean. If the right side carb isn't getting enough fuel into the carb (which I don't think is the issue because when I pull the float bowls there appears to be the same amount of fuel) how would I change that? Run the float at 18 instead etc? I also plan on replacing the pistons and getting some headwork this winter I am just trying to make her good for the huge toy run for christmas. Thanks.
 

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NOT the mixture screws... The IDLE stop screws! (on the "hammer" arms, which will/might involve adjusting the cables as well).... Your mix screws sound about correct, although you may have to tweak them a smidge.... Keep float settings at spec.....
I'll mail the header clamp as soon as i can get the *!$%!* bolt free....LOL.... :lol: Steve
 

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Discussion Starter · #5 ·
Update....I tried adjusting the mixture screws both ways. It seemed to not pop quite as much when the right side carb was leaner than the left but still did pop. When I timed it for 4,000 rpm my 1,100 was hitting on the lt mark instead of lf. Exhaust gas on the right side is still cold at idle but at about 3,000 heats slightly. Left side can was emitting a little black smoke and none out of right side.
 

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The "playing" with timing has solidified that that likely isn't the main problem...... I trust you have re-timed it to exact on both LF and F marks.....
Again, NOT the mixture screws, ...please adjust/sync the idle screws......
I still believe you have one side's idle set so high that the other side is nearly literally shut-off (except for the slight fuel/air supply through the mixture passage) to compensate and achieve the relatively low 1100 idle...... You could try simply turning the IDLE (not mixture) screw up on the "cold" side to get a 1300 steady idle.... then back the "hot" side down to reduce/correct it back downwards...
 

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Discussion Starter · #7 ·
I think you hit it on the head. At it's current setting if I adjusted the air screw all the way in on the right side then the bike kept running. Only if I backed it out 3 turns did it affect anything. Now I adjusted the idle on the right side up and the right side down a little and when I adjust the air screw it actually affectedd it. The only problem now is getting the 2 dialed in. Right now when all seems good the idle is sooooo high. Then when I give it gas it isn't coming back to idle soon. Once I adjust the idle screws down it starts the popping again but does better about returning to idle. God I hate dual carbs!!!!!!! :evil: :lol:
 

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Discussion Starter · #8 ·
So first off thanks for the help. I rode it today and she is running better than ever. The only odd thing is the rpms stall upon decel then settle at idle. If I accelerate the left carb only the rpms come right back but if I do the same with the right side carb it comes down but stalls at about 2500 then slowly goes to idle. My first thought is the slide might be sticking a little. That or an air leak at the slide gasket. Just wanted thoughts before tearing it apart. Thanks.
 

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owdskl said:
I think you hit it on the head. At it's current setting if I adjusted the air screw all the way in on the right side then the bike kept running. Only if I backed it out 3 turns did it affect anything. Now I adjusted the idle on the right side up and the right side down a little and when I adjust the air screw it actually affectedd it. The only problem now is getting the 2 dialed in. Right now when all seems good the idle is sooooo high. Then when I give it gas it isn't coming back to idle soon. Once I adjust the idle screws down it starts the popping again but does better about returning to idle. God I hate dual carbs!!!!!!! :evil: :lol:
If your "air screws" don't dial in at around 3/4 turn or so your carbs are dirty........having to turn them all the way in or 3 turns out is a sure signal.
 

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Discussion Starter · #10 ·
Both had best rpm at about 1 turn out. Carb have been soaked rinsed and blown out as well as all new jets and passed the straw test. Although I didn't pay attention to the rate In which the slides came back down. I can try pulling that right side carb again and blow it back out.
 

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owdskl said:
Both had best rpm at about 1 turn out. Carb have been soaked rinsed and blown out as well as all new jets and passed the straw test. Although I didn't pay attention to the rate In which the slides came back down. I can try pulling that right side carb again and blow it back out.
You said in your prior post that you had to screw it out three turns......
 

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Discussion Starter · #12 ·
The three turn out comment was before I fixed them right now I get changes at almost any turn of the air screw. Right now they are both at 1 turn out. Sorry for the confusion.
 

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You did retime the points so they fire at EXACTLY the LF and F marks and the gaps remained within spec when/after you were "playing" with them...Correct?
 

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Discussion Starter · #14 ·
Yes I did. I would like to say that it is running badass except for the stall out.
 

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On the slides/pistons, are the center 'stems' completely chrome, or is there "wear" down to brass?...
Is there any possibility the domes were put on "rotated" or pistons were "Swapped"?
 

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Discussion Starter · #16 ·
No chance of a swap of them. I don't recall about the chrome or brass but I will check. I would assume that means they could stick because of wear.
 

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steve,
brass would indicate excessive wear and somethings wrong? at that point your suggestion would be new carbs? i've got some minor wear, a little brass showing at the bottom edge. sized less than 1/2 of a dime. that said, mine still seem to slide OK.

seth
 

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If your right side carb has a crack in the float then you are running too much fuel which will cause it to run cool and backfire.

you didn't mention if you had fixed the float or not but assuming you haven't this would explain everything you have described.
no amount of tuning will correct your problem if you have a waterlogged float. If you have already addressed the float issue AND RESET THE PROPER FLOAT HIGHT. THEN I'd follow the rest of the great suggestions above.
 
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