Hey guys just bought a worked 4k. it has a 270 mild cam, double valve springs, oversize flat top pistons, ported and polished head with toyota twin carbs match ported.

Problem is that the engine doesnt have much manifold vaccum at idle and isnt running the best.

any ideas?

 

also can someone tell me what jet sizes i should b using for the carbs? the port sizes arnt huge but are quite big...

cheers Brock

oh and it isnt the carbs as i chucked the single carb off my old 3k on it to check

timing is overrated

quite stumped really

You are tapping the vacuum off the manifold itself, and not the vacuum advance line to the carb?

yeah getting vacuum from the inlet manifold itself

Cam Timing??

 

Vacumn leak at manifold???

 

Get a can of crc and spray it around the manifold, if the engine starts to run better, you have a leak.

Cam Timing??

 

Vacumn leak at manifold???

 

Get a can of crc and spray it around the manifold, if the engine starts to run better, you have a leak.

 

checked the timing marks on cam and crank and they line up.

have used 2 different manifolds and the gaskets are new...

A good easy way to check cam timing is to remove the rocker cover. When you turn over the motor to no.1 on TDC firing, the valves for no.4 should be "on the rock" (ie exhaust closing, inlet opening).

Edited August 29, 200917 yr by Felix

A good easy way to check cam timing is to remove the rocker cover. When you turn over the motor to no.1 on TDC firing, the valves for no.4 should be "on the rock" (ie exhaust closing, inlet opening).

 

 

checked all the basics like that. have done compression tests on all cylinders and even don leak down tests and its overrated. a mild grind cam shudnt affect manifold vacuum tho shud it?

Its' always best to check the basics first. :y:

 

I run a 270 degree cam. It doesn't affect vacuum that much. On my old 4k just before I pulled it out, it gave 160 psi compression with a 9.8:1 CR after a good 4years of flogging.

Open the inlet tappet gap to close the inlet valve earlier and see if that makes a difference. Check all the gaps while you're at it... You never know... smaller gaps mean a wilder cam and a rougher idle.

 

The stock cam timing marks don't mean a lot with ground cams, and if the tappet gap makes a difference you may need to retard the cam a couple of degrees. We drilled more holes in the cam wheel on our rally Datsun to give a wider spread of cam timing, and that worked well.