620 sputtering

[sergio620]

Hello I got a 620 with l20b dual hitachi flat top su's and started experiencing some sputtering as I was leaving work. As I turned on the truck it was idling and for a bit the rpm would jump from around 600 to 700 repeatedly. As I started driving, to the truck was sputtering and acceleration felt weak. Any help would be appreciated thanks

[racerx]

Have u checked for vacuum leaks and/or when was the last time u tuned it up.

[flatcat19]

Suck, squish, bang, blow.

 

Something is amiss. What have you tried?

[datzenmike]

Probably vapor lock. It gets hot under the hood and the gas will boil in the fuel lines and in the carb. You should fill out your profile to include the year of 620. If pre '75 it doesn't have a return line to keep cooler gas circulating.

 

Try making heat shields for the intake.

[banzai510(hainz)]

I start with timming and ck the ignition especially if still running points is the weak spot , dist shaft wiggle will cause something like this.

I know nothing about SUs

 

cheacking valve lash is also simple.

[sebpv]

Mike you mentioned the no return on pre '75s. What is again that line connected on the driver side all through to the fuel tank? Just a vent tube? Can I delete it? On my '73 btw...

 

EDIT: Found my answer; its the line for the charcoal canister. So useless without the full emission system in place

[datzenmike]

 The gas cap is sealed so as fuel is used, air passes down the tube into the tank.

 

If there is any evaporation or expansion in the tank when parked the fumes force their way to the flow guide valve on the inner fender and from there into the engine vent tube to be stored in the crankcase. Next time the engine starts the PCV valve draws all the fumes into the intake to be burned.

 

Best if this is left running, it does no harm and has zero effect on running.

[sebpv]

what is your opinion on a breather cone directly connected to the tank? I sorta wanted to rid the engine of as much lines as i could without hurting the engine

[datzenmike]

Well fumes will get out and you need the tank vented. Easier to just leave it.

 

I suppose you could find a vented gas cap and just seal the line. Fumes can get out and air can get in if needed. If the cap ever gets plugged you risk imploding the tank. Done that before.

[sebpv]

Then its settled, Ill get the gas cap. Thing is my truck is in pieces. So Id rather NOT put a ton of lines back on.