bogging engine

[evildky]

this is my first post to this board so be gentle,

I have a 71 240z origional engine, it has had a problem since I got it a year ago, once it gets good and warmed up, it starts bogging down, I can be racing it up to 5k in secong and then it just starts to bog for no reason, I can upshift to third and it'll bog at about 3500, up to forth and it's at like 2k, I have replaced the SU's with a set from an IT Car, I replaced the point dist with a 78 electronic and an aftermarket coil with an MSD 6t, the problem is still there, I have aljusted the valves, swithed to an electronic fuel pump with an adjustable FPR, I am out of stuff to replace, all my plugs look good, just a light coat of light dusty brown evenly all over the surface of the plug, guessing from the msd box, anyone got an idea? I am really needing to get this thing fixed

[TTDRIVER]

Do you have aftermarket headers on it? Does it have a heat shield? Maybe its because your headers get so hot that the heat makes the fuel in the carbs evaporate. Im not to sure but I know that it is possible.

[evildky]

I do have headers now, I also still have the heat shield under the SU's, but the problem dates back to when it had factory intake and exhaust, any other ideas?

[TTDRIVER]

Does it over heat at all? Who tuned the carbs?

[evildky]

it does not over heat at all, the guage rises and fluxuated around the half way mark, I tuned the caabs, I have tried lean, rich and everything in between, on both sets, I have checked the float levels, and nothing seems to work

[evildky]

it idles great and runs fine till it gets good and warmed up, then the bogging keeps it from reving up but it still idles fine

[TTDRIVER]

I dont know what to say. My car when ideling would start to overheat do to a blown headgasket. As it got worse it would start to bog out just like you mentioned. What about your fuel pumps? How is the mechanical one? maybe its starving for more gas. Have you checked the fuel filter? If that dosent help im sure someone here will help you find the problem.

Good luck!

[evildky]

yeah the fuel pump occoured to me, I now have a holley red pump with an adjustable fpr, no improvement, thanks for trying

[Gav240z]

How much PSI is the fuel pump rated at?

How are your fuel lines are they ok condition wise?

Sounds like too much fuel if it bogs. Do you get smoke out the back?

Best way is to open throttle at night infront of another cars headlights .

You have got a new set of SU's what condition are they in?

The seats could be worn in the carbs allowing too much fuel in even if you set them leaner.

Check the throttle shafts if you spray carb cleaner around them does the engine revs rise? If so the throttle shafts need to be rebushed as they are leaking causing incorrect balance.

Perhaps try driving around with the bonnet slightly open so it will let the heat out does it still act up?

How about the dashpots do they rise nice and evenly if not that could cause problems.

If none of those we will try your ignition set up.

Also could help alot of you provide a pic of your engine bay and it's current set up.

Cheers.

Gav,

[Gav240z]

Just a final thought the IT car you got the carbs from was it 2.8ltr or 2.4ltr?

If 2.8ltr the needles and seats could have been changed to accomodate for the bigger engine size resulting in a rich mixture.

Most likely choice would have been SM needles from Ztherapy.

Try get a hold of a normal L24 size needle and seat for each and try that.

Gav.:cheeky:

[evildky]

the holley pump is rated for 5 or 7 psi can't remember, I had to add an adjustable frp to keep gas from coming out the fuel bowl vents

the car is an early 71 and the fuel lines are in the trans tunnel, they look fine at both ends

it only smokes when I first fire it up, just a bit for the first coupla minutes

the carbs looked real clean, one had a clog in the bottom of the fuel bowl, resolved that problem the first time I fired them on the car

I am guessing the carbs came from a 2.8, why would someone race a 2.4 in it when they could rave the 2.8?

so I am sure that it's rich, but this problem pre dates the carb swap ( which I though might fix the problem)

I have sprayed carb cleaner everywhere around the intake and carbs including the throttle shafts, balance tubes, cylinder head, and vac hoses (both of them)

I have no dashpots, and the needles are flat faced, so yeah very rich, but it runs fine for the first 10-15 minutes, then nice and heated up if starts bogginf, and yeah I tried with the hood popped, no change

I'll try and get a pic tonight,

keep the ideas comin, thanks

[2ManyZs]

If you still have the original gas tank, run it down as low as you dare, then pull the drain plug. If you get a lot of rust, scale and garbage out, I'd bet the pickup is getting blocked with garbage in the tank.

When you rev it up to high rpm, you are drawing fuel and whatever gunk might be in the tank up to the pickup tube, then, it will stay there until you turn the car off. Since you don't say that the car runs erratically after you shut it off and let it sit a while, that would be one of the most likely problems.

If you haven't done it already, block off the water line to the intake and see if that makes any difference.