Bike cutting out after Warm Up post rejet.

[tom4130]

I have a 80 Special that I am currently working on to build a street tracker kind of build.

I have recently ditched the stock air box in favor of UNI Pods, and also added some shorter reverse megaphone mufflers to my build. The bike was running fine before I did this so my thought was that with the higher air flow, I would need to rejet the carbs.

I went up one size on both the pilot jets and the main jets, and with the choke on, the bike fires up within a couple of kicks, and stays running for a minute or so before dying.

From here I have played around with the choke open, closed, Half open and the bike simply will not fire again. It seems that the bike gets to a certain temperature and then just cuts out.

Should I go back to the stock jets? or am i not getting enough fuel? Should I go up another size on the main jets? I am just a little confused as the bike was running great as I bought it, and yes I have had it apart, but it seems to be fine until it is warmed up.

Thanks everyone for any help on this.

 

[TwoManyXS1Bs]

Is it still running TCI? If so, you can try the forum's google search on 'rare earth magnets'. Many of the TCI users here have noticed ignition problems related to weakening trigger magnets, and have come up with some interesting tests and fix ideas...

 

[retiredgentleman]

Does the bike still have the stock fuse panel?

Do you have a VOM and have you measured the voltage at the TCI input and at the ignition coil input?

 

[hooser]

Check for air leak from engine to carb. Spray wd40 in those areas. Might be sucking air. so closing the choke will reduce the extra air. Intakes my have gotten loose when you removed parts.

 

[tom4130]

@TwoManyXS1Bs It does have the TCI, however Im not sure that is the problem, only a couple of months ago i was doing 50 mile trips regularly, I was under the impression that this would be a carb / fuel problem and not electrical since it starts up just fine.

Does the bike still have the stock fuse panel?

Do you have a VOM and have you measured the voltage at the TCI input and at the ignition coil input?

@retiredgentleman I do have a VOM but no I have not yet measured the voltage at the TCI. I am still running the stock fuse box. Should I be looking at getting 12 - 13 V at the input to the tci? Sorry I'm a bit of an electrical moron. Which wire should I be measuring?

As I put above, I was under the impression that this would be a fuelling issue seeing as I rejetted, but is this a more common issue?

Thanks so much both of you for the help.

 

[tom4130]

Check for air leak from engine to carb. Spray wd40 in those areas. Might be sucking air. so closing the choke will reduce the extra air. Intakes my have gotten loose when you removed parts.

Will do thanks @hooser.

I'm assuming I am looking for bubbles of air on the WD40?

Im just confused as the bike starts up, then idles and dies, then I cannot get it started at all again, even with the choke closed until the bike has cooled again.

Thanks again.

 

[hooser]

I was just going by you saying it ran ok until you messed with the carbs. You will notice a change in idle as you spray. I just had one about the same..

 

[tom4130]

I was just going by you saying it ran ok until you messed with the carbs. You will notice a change in idle as you spray. I just had one about the same..

Gotcha, thanks man Ill give that a go and see what I see.

 

[Brassneck]

Do you have enough fuel? a minute of running and then not could be the fuel supply is low... how much fuel is in your tank? Try turning the petcocks to prime...then running the bike...see if it dies as you described... if it stays running...turn them back to the "on" position. If so, you either don't have enough fuel in your tank or the flow is the issue.

 

[tom4130]

Do you have enough fuel? a minute of running and then not could be the fuel supply is low... how much fuel is in your tank? Try turning the petcocks to prime...then running the bike...see if it dies as you described... if it stays running...turn them back to the "on" position. If so, you either don't have enough fuel in your tank or the flow is the issue.

Brassneck thanks for the post, Yeah I was actually putting the petcock into prime to fill the floats and then starting it and turned it to 'on' straight away. I do have enough gas in the tank but it could be that the 'on' position is not sending fuel through the petcock.

I need to read a little more into how the stock vacuum petcock works and see if that is the issue. Thanks

 

[Brassneck]

OK, so keep it on prime and run the bike (Meaning don't turn it to "on"), essentially this eliminates the petcock altogether and allows fuel to flow straight to the carbs...and see if the bike runs without cutting out. If it doesn't cut out, then there's the issue.

The vacuum line essentially pulls opens the diaphragm in the petcock allowing fuel to flow out as the piston creates "vacuum" in it's cycle. (In either the "On" or "Reserve" setting only), One way to test the vacuum function is to make sure they are turned to "ON" or "Reserve" and pull the fuel line off the petcock and pull the vacuum line off the carb holder... hold a cup under the petcock and simply suck on the vacuum line. It's only air your sucking on... and might taste foul, but if the petcocks are working, gas will flow (doesn't take a lot to get it to open up).

If no gas, and you're sure the tank is full...then you have a petcock/vacuum issue.

 

[Brassneck]

Funny note: I recently put my tank back on, and had the same issue...started, but would die after a min. I thought it was all sorts of things...but didn't realize the gas I poured in the tank wasn't enough to flow through the "On" setting... I didn't think to turn it to "reserve" and was getting ready to take apart my carbs. I decided one last try with it set to prime, and it ran great... back to "on" and it would die after a min... switched to reserve, and it ran great again... hope your solution is as easy as a trip to the gas station.

 

[tom4130]

OK, so keep it on prime and run the bike (Meaning don't turn it to "on"), essentially this eliminates the petcock altogether and allows fuel to flow straight to the carbs...and see if the bike runs without cutting out. If it doesn't cut out, then there's the issue.

The vacuum line essentially pulls opens the diaphragm in the petcock allowing fuel to flow out as the piston creates "vacuum" in it's cycle. (In either the "On" or "Reserve" setting only), One way to test the vacuum function is to make sure they are turned to "ON" or "Reserve" and pull the fuel line off the petcock and pull the vacuum line off the carb holder... hold a cup under the petcock and simply suck on the vacuum line. It's only air your sucking on... and might taste foul, but if the petcocks are working, gas will flow (doesn't take a lot to get it to open up).

If no gas, and you're sure the tank is full...then you have a petcock/vacuum issue.

Perfect explanation thanks so much. If I run the petcock on the 'ON' position will I not overfill and flood the float bowls or will the diaphragms still stop the fuel flow once the floats reach a certain height?

If so, then what is the point in the vacuum? Sorry I am sounding completely clueless here but I had a CB350 that was gravity fed and was fine.

Thanks Brassneck, really helpful I will see if I have a petcock problem and report back.

 

[Brassneck]

Your carbs (assuming stock) are gravity fed as well...but the vacuum petcocks were designed to not have to deal with an On/Off style petcock...so you could simply turn the bike off and walk away. (Or in an extreme case, if you crashed, and the bike stalled, then the fuel would stop flowing as well.)

No pistons moving= no vacuum = no gas flow.

Setting them to "On" or "Reserve" requires vacuum to have gas flow... setting it to Prime means gas flows free...just like the "On" setting on your CB... with the bike running, the floats should do their job and prime shouldn't flood the carbs....and for this, all we're doing is testing the ability to keep your bike running, eliminating any question about fuel flow... But after this test, you don't leave it on prime because if the bike dies, then yes...theoretically, the gas will keep flowing.

Hope that makes sense.

 

[tom4130]

Hope that makes sense.

Gotcha, cheers man that helps. If its as simple as that and I've been stressing over nothing I'm gunna look a right tool

Away from the bike until the weekend but will try just adding gas.

Cheers mate.

 

[tom4130]

hope your solution is as easy as a trip to the gas station.

@Brassneck, you are the man. Filled her up a little more and she started up and kept going. Havent been for a ride yet but ran her on the 'Prime' Setting for about 5 mins with no issues.

I'm assuming this means the vacuum 'on' setting is gone on my petcock then. Is it a matter of just cleaning it out or is there something else I should be looking for?

Thanks so much for the advice. Feel a little bit stupid that the problem staring me in the face all along

 

[angus67]

don't feel stupid about it. a lot of guys switch to standard petcocks for the same reasons.

 

[Brassneck]

Filled her up a little more and she started up and kept going. Havent been for a ride yet but ran her on the 'Prime' Setting for about 5 mins with no issues.

I'm assuming this means the vacuum 'on' setting is gone on my petcock then. Is it a matter of just cleaning it out or is there something else I should be looking for?

Thanks so much for the advice. Feel a little bit stupid that the problem staring me in the face all along

Awesome! And definitely don't feel dumb...like I said, happened to me, and drove me nuts just a few weeks ago.

Anyways, Glad you got it to stay running and at least you know it's a flow issue. so I would do a little more testing before pulling the petcocks...how much gas is in your tank? I'd again see if the added gas allows you to keep it running in the "reserve" position...if so, then try "on" position...if neither of those wotk, then I'd pull the fuel line and manually suck on the vacuum line to see if fuel doesn't actually come out.

And if they dont, then taking the petcocks apart would be required...i woukd look for blocked passages (taking the cover plate off first, as that's where the fuel pathways are. Perhaps the rubber gasket has blocked the "on" pathway?

Hard to know for sure...

Oh, and don't forget to turn the petcocks to "On" when the bikes off, so you stop the fuel flow.

 

[tom4130]

Thanks guys,

yeah great @Brassneck, Ill check it all out but at least I know where the problem was stemming from. Thanks for the help

 

[5twins]

I've encountered this issue a few times over the years, and with non-vacuum petcocks too. It seems I would finish a ride just as the bike needed to be put on Reserve, but didn't know it. The next time I went to start it, it would start and stall, or not start at all. I was out of gas and didn't realize it. Try Reserve any time you encounter a situation like this. You never know .....