Honda Twins banner

1 - 20 of 29 Posts

·
Registered
Joined
·
41 Posts
Discussion Starter #1
Ive got a 73’ 350 scrambler that runs like a champ until it gets 40-50 miles on the trip odometer. At that point I’ll experience surging and sputtering- switching the petcock to reserve provides no relief. Tank has little to no rust and the fuel lines have filters installed after the carbs were rebuilt. Stopping at the gas station and filling up again and she runs like a champ again. Clogged petcock? The next phase of the mechanical restore The plan was to reseal the tank and rebuild the petcock Might need to move that up if that is the case. What do you think?
309662
 

·
Super Moderator
Joined
·
10,202 Posts
Try removing the fuel filters. If the strainer/standpipe on the petcock is in place you don't need them anyway.
 

·
Registered
Joined
·
13,085 Posts
If it was a clogged petcock it would not be solved by re-filling the tank. Since reserve doesn't make a difference it might be the cap vent. I know it goes much longer than usual in miles before it does it, but if the vent is just starting to become clogged that might be all it is. Next time it does it, pop the cap and see what happens. Or, depending on the type of filters you're using, it could be those too, some don't flow as well as others and yours might need the extra fuel bulk to have the "weight" needed to flow enough
 

·
Registered
Joined
·
2,819 Posts
Pull the hoses off the carb inlets and run some fuel into a cup. You should be able to see if there's enough flow there to supply the engines's needs. Don't forget to check out the vent in the gas cap for clogging, that will affect fuel flow also.
 
  • Like
Reactions: JamesPal

·
Registered
Joined
·
41 Posts
Discussion Starter #5
If it was a clogged petcock it would not be solved by re-filling the tank. Since reserve doesn't make a difference it might be the cap vent. I know it goes much longer than usual in miles before it does it, but if the vent is just starting to become clogged that might be all it is. Next time it does it, pop the cap and see what happens. Or, depending on the type of filters you're using, it could be those too, some don't flow as well as others and yours might need the extra fuel bulk to have the "weight" needed to flow enough
Thanks Ancientdad. I’ll look in the manual on how to separate from the cap lid- appears to be spring loaded. I know my gasket is in need of a change as well.
309672

309673
 

·
Registered
Joined
·
854 Posts
Your problem is how you ran the fuel lines, you are forcing the fuel to run uphill. When the tank is down a gal or so there isn't enough pressure to push the fuel to the carbs. Shorten up the lines, get rid of the inline filters and see if that fixes it.
 

·
Registered
Joined
·
2,819 Posts
Doesn't matter, the fuel in the tank is still higher than the carb inlet. Once the hose is filled the gas has to run out the end.
 
  • Like
Reactions: colvfrfg

·
Registered
Joined
·
854 Posts
I'm sure you're right Mike but I would still change the way the lines are run.
 

·
Sensei
Joined
·
27,023 Posts
The problem is almost assuredly the sintered bronze filters....They require fuel pressure to adequately flow and the gravity /weight /height of fall pressure obviously reduces enough @ 1/2 tank to prevent that.....That type of filter is fine in a system with a fuel pump, but not so much in gravity fed systems.....
 

·
Registered
Joined
·
854 Posts
66Sprint, does this mean i was right even though I was wrong?😁
 

·
Registered
Joined
·
41 Posts
Discussion Starter #11
Great intel guys. The lines and filter were professionally installed so I‘ll have them review and swap with a more compatible filter. I’ll swap out the gasket and check the vent next weekend. Thanks for all your help.
 

·
Registered
Joined
·
41 Posts
Discussion Starter #13
Hmmm... another vintage shop that shouldn't be? Hope not, we hear enough about that type
Yea I wish I had the time but work puts me on the road a lot. I have had good luck with these guys to date- Atlanta Motor Works
 

·
Registered
Joined
·
2,509 Posts
+1 on the inline filters are definitely the issue.

If your tank is clean then you can stop the use of an inline filter.

Your petcock reserve feed may have a strainer, and the petcock bowl may have another strainer.
 

·
Registered
Joined
·
3 Posts
+1 on the inline filters are definitely the issue.

If your tank is clean then you can stop the use of an inline filter.

Your petcock reserve feed may have a strainer, and the petcock bowl may have another strainer.
Based on my personal experience, I agree with just about all of the above advice.
(1) check air flow at gas cap (probably still OK)
(2) remove in-line filters (not really needed on a relatively clean tank, built in sediment bowl in petcock is sufficient)
(3) Shorten fuel hoses. This combined with (2) above are your main issues. The fuel flow rate through the petcock when tank is full, compared to when tank is below half-full, is dramatically different. Try yourself. Run a stopwatch and compare how much fuel flows out (you need a measuring cup) in 1 minute when tank is full, compared to below 1/2 tank. Both times run it on reserve so it is an accurate comparison.
 

·
Registered
Joined
·
6 Posts
I’ve run those filters on all my vintage bikes without any issues. My CB350 cafe is running at reserve levels and no issues. Does the bike run again if you let it sit for a bit? Do you look in the tank to see your fuel level? Before removing things and cutting lines. I would check that first. If you really don’t have fuel or super low, changing your line set up is a waste. Also, if you check fuel, then start back up, and the bike runs fine again it could be your gas cap not venting well, almost like a straw creating a vacuum.
 

·
Registered
Joined
·
19 Posts
+1 for inline filters, same exact issue on my 350 - but it could also be related to your petcock. There are cheaper versions of this petcock for sale in lots of places, and the shop I bought mine from replaced the original with a knock-off of sorts that did not flow well after 1/2 a tank.

While the physics of line set up may not matter in theory, it takes literally 2 seconds to get them to the right length.
 

·
Registered
Joined
·
5 Posts
It sounds to me like the vent on your gas cap might be plugged up. The next time your bike starts to lose power, pull over and open the gas cap. If you hear a rush of air entering the tank like you just opened up a vacuum, it's because you did. Try starting the bike up right after you opened up the gas cap. If it runs without losing power again, you found the problem.

The tank needs to be able to vent to allow gas to flow out to the carbs, like putting your finger over the end of a straw when it's in a drink. You pull the straw up and the drink stay in the straw until you let your finger off the end.
 

·
Registered
Joined
·
1 Posts
I have a ‘69 CB350 and ‘65 Bomber and have run into the same problem on many old Honda’s. Do the following: If you’ve never removed your petcock to check it, the reserve inlet pipe inside the tank can deteriorate and snap off or develop rust holes and leak sediment and build up in the reserve channel inside the petcock blocking fuel from getting to the carbs when the gas drops below the main petcock level in the tank. This effectively eliminates your use of the reserve level of gas, that’s why your bike starts running bad after your over a 1.5 gallons into your ride - your running out of gas! This is easy to
Check, just disconnect your fuel lines and see if gas is flowing in the “on” position, and then the reserve position. If your reserve has no gas coming out of it, it is clogged and you need to remove the petcock and clean it with carb cleaner and blow air through it until it flows freely like the main position.
 
1 - 20 of 29 Posts
Top