Backfire through carb
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Stage III
Joined: Jul 2014
Posts: 3
Likes: 0
From: Ss
Backfire through carb
Hey guys I'm new here... First post, I'm pulling my hair out because like the name says I'm trying to learn the old stuff..... I have a '61 panhead and its backfiring through the carb... I just picked this bike up and it had an "SU" on it which i was told was junk and that was the problem so i changed it out to an old kehien and its still doing it. Ive sprayed and sprayed wd-40 and the motor doesn't react...i also put a set of the wide band seals on it. I know with it backfiring in carb it is probable that it sucking air but it just isn't showing up... Wondering what else to try... I loosened the stud on the distributor to adjust timing but the band is stuck and will not move, plugs have equal carbon build up... I thought if it was leaning out and sucking air one would be cleaner... Any input is appreciated... Smart asses are not....like i said I'm trying to learn this lost art.
Seasoned HDF Member
Joined: Nov 2006
Posts: 12,878
Likes: 5,449
From: la la land jerzey
pans are not efficient and you did not say what vintage Kehien carb early ones use a large low speed jet 85 in some cases and the later carb uses a 42 and one jet size can make a difference and that is 40 sizes-
its not a lost art its paying attentiion to the small details 61 pan had a linkert carb that is very easy to tune --
ANYWAY you need to perform basic engine tune and checks before an answer can be made the guys on this site are very helpfull and unlike the bozos on the other sites ride and fix this sort of machine and can help
you need to check the valve adjustment
check the points and replace them AND the condenser its all china junk today
replace the sparkplugs and wires
do a compression test YEA i know its a kicker - you want to see what ever the number is - is it the same not 20 lbs different
a leak down test is even better in some cases and this could be one as it will show a valve not seating or way to much compression loss into the crankcase
if you do not have them - buy the repair and parts books as the correct part name on this site will help all understand what your working on
its not a lost art its paying attentiion to the small details 61 pan had a linkert carb that is very easy to tune --
ANYWAY you need to perform basic engine tune and checks before an answer can be made the guys on this site are very helpfull and unlike the bozos on the other sites ride and fix this sort of machine and can help
you need to check the valve adjustment
check the points and replace them AND the condenser its all china junk today
replace the sparkplugs and wires
do a compression test YEA i know its a kicker - you want to see what ever the number is - is it the same not 20 lbs different
a leak down test is even better in some cases and this could be one as it will show a valve not seating or way to much compression loss into the crankcase
if you do not have them - buy the repair and parts books as the correct part name on this site will help all understand what your working on
Last edited by johnjzjz; Jul 16, 2014 at 09:04 AM.
Road Master
Joined: Feb 2008
Posts: 1,012
Likes: 1
From: SoCal
Everything that johnjzjz said.
I'll only add that I had the same problem with my Pan, went through a lot of steps and it still would spit back at me...especially when kicking. My problem was timing.
I had a POS auto advance circuit breaker on the bike. I couldn't get it timed right. I tried everything. Finally I went all in and broke out the timing light. I found that the timing was jumping all over the place...stupid sprung weights weren't consistent.
I took my time and found an OG manual advance circuit breaker in good condition with USA made points and condenser and put it on the bike. It's a totally different machine now. Starts on the first kick, idles right, and purs on the freeway.
Just keep troubleshooting and you'll find your problem. Try to replace the repop stuff with good used OG parts as you go, that will help too.
Find a Linkert, you'll probably need a m74b...your Pan will thank you!
I'll only add that I had the same problem with my Pan, went through a lot of steps and it still would spit back at me...especially when kicking. My problem was timing.
I had a POS auto advance circuit breaker on the bike. I couldn't get it timed right. I tried everything. Finally I went all in and broke out the timing light. I found that the timing was jumping all over the place...stupid sprung weights weren't consistent.
I took my time and found an OG manual advance circuit breaker in good condition with USA made points and condenser and put it on the bike. It's a totally different machine now. Starts on the first kick, idles right, and purs on the freeway.
Just keep troubleshooting and you'll find your problem. Try to replace the repop stuff with good used OG parts as you go, that will help too.
Find a Linkert, you'll probably need a m74b...your Pan will thank you!
Seasoned HDF Member
Joined: Oct 2005
Posts: 7,495
Likes: 3,705
From: Santa Klaus County, Cali
Everything that johnjzjz said.
I'll only add that I had the same problem with my Pan, went through a lot of steps and it still would spit back at me...especially when kicking. My problem was timing.
I had a POS auto advance circuit breaker on the bike. I couldn't get it timed right. I tried everything. Finally I went all in and broke out the timing light. I found that the timing was jumping all over the place...stupid sprung weights weren't consistent.
I took my time and found an OG manual advance circuit breaker in good condition with USA made points and condenser and put it on the bike. It's a totally different machine now. Starts on the first kick, idles right, and purs on the freeway.
Just keep troubleshooting and you'll find your problem. Try to replace the repop stuff with good used OG parts as you go, that will help too.
Find a Linkert, you'll probably need a m74b...your Pan will thank you!
I'll only add that I had the same problem with my Pan, went through a lot of steps and it still would spit back at me...especially when kicking. My problem was timing.
I had a POS auto advance circuit breaker on the bike. I couldn't get it timed right. I tried everything. Finally I went all in and broke out the timing light. I found that the timing was jumping all over the place...stupid sprung weights weren't consistent.
I took my time and found an OG manual advance circuit breaker in good condition with USA made points and condenser and put it on the bike. It's a totally different machine now. Starts on the first kick, idles right, and purs on the freeway.
Just keep troubleshooting and you'll find your problem. Try to replace the repop stuff with good used OG parts as you go, that will help too.
Find a Linkert, you'll probably need a m74b...your Pan will thank you!
Road Master
Joined: Feb 2008
Posts: 1,012
Likes: 1
From: SoCal
The shop were I got this from said the same as you...that you just can't find these parts made in the USA anymore.
Seasoned HDF Member
Joined: Nov 2006
Posts: 12,878
Likes: 5,449
From: la la land jerzey
flea bay - but i have been buying the different electronic ignitions that are available that bolt in place instead of the points - when i find and use a magnetic pickup that i like i will be on about it
Thread Starter
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Stage III
Joined: Jul 2014
Posts: 3
Likes: 0
From: Ss
Thanks for the input guys.... Ive piddled with it some and everything im trying so far is helping a little but ive about narrowed it down to a timing issue. I believe my distributor has some slop in it and the play is causing problems im going to change out this week and will update yall. Thanks again for the inputs so far
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Stage III
Joined: Jul 2014
Posts: 3
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From: Ss
Looks like I've got a intake valve not seating... So will be pulling apart soon, thanks for everyones help
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From: Phoenix '53, '88, '09 Big Twins
easy to check this by introducing compressed air into the cylinder through spark plug hole.
on comp stroke ( both valves closed) you shouldn;t hear any air through intake or exhaust.
you can rotate the motor and note when the air comes from exhaust or intake referencing to the timing marks on flywheel.
this will give you an idea of valve seal, cam timing.
sneeze through the carb is usually a lean mixture or too advanced timing- so if you have ruled those out proceed.
the SU can be a great carb as it is a constant velocity type- if your keihin is a 1990-2006 type Keihin CV you have an upgrade...if you have the earlier keihin butterfly carb...oops.
for accurate timing you need the gear lash to be correct for the cam, idlers and timing gears.
I use an EBeyond 2000 sensor- it works with 6 or 12 volt systems and is maybe the best money I've spent on upgrading my FLF--- in the past I had problems with crap condensors allowing my points to arc and pit.
Not no more. install takes minutes, uses stock coil
Mike
on comp stroke ( both valves closed) you shouldn;t hear any air through intake or exhaust.
you can rotate the motor and note when the air comes from exhaust or intake referencing to the timing marks on flywheel.
this will give you an idea of valve seal, cam timing.
sneeze through the carb is usually a lean mixture or too advanced timing- so if you have ruled those out proceed.
the SU can be a great carb as it is a constant velocity type- if your keihin is a 1990-2006 type Keihin CV you have an upgrade...if you have the earlier keihin butterfly carb...oops.
for accurate timing you need the gear lash to be correct for the cam, idlers and timing gears.
I use an EBeyond 2000 sensor- it works with 6 or 12 volt systems and is maybe the best money I've spent on upgrading my FLF--- in the past I had problems with crap condensors allowing my points to arc and pit.
Not no more. install takes minutes, uses stock coil
Mike
Last edited by mkguitar; Jul 30, 2014 at 02:50 AM.
