Truck falls on its self at 4500RPM???

[largekid]

even in Neutral it falls on its ass at 4500 RPM, the tach jumps arround and it sputters like crazy..

im thinking either to low of fuel preasure or the secondarys arnt opening..

any other ideas to try?

it runs perfect untill 4500...

[Kyle]

number one spot to start trouble shooting a fuel starvation problem....fuel filter

[ZZ4Blazer]

Coil? Ignition system?



I don\'t think it would be the fuel system if it does it without any load, like in neutral.

[adh383]

i\'m not much on electrical stuff but i think if you were running out of fuel at 4500 -your rpm\'s may drop but dont think it would make your tach jump all around. Plugs ok? wires? just an idea!

[Scotty_S-15]

Any chance it\'s as simple as this?

.............. Last year I met a guy in the staging lanes at the track, he was complaining about the same thing, but I think he was dropping dead @ 5K... So I\'m looking under his hood as we talk, and I\'m looking at his MSD box, I see a limiter chip sticking out, and I ask him what chip he has... his reply.... what chip? http://www.s10v8.com/images/smilies/icon_razz.gif Yep, it was a 5K chip. He never knew it had a rev limiter. Sure made HIS day! http://www.s10v8.com/images/smilies/icon_biggrin.gif

[Kyle]

i don\'t think a rev limiter or an ignition problem would cause the tach to jump around...sounds like fuel surges.

[ZR1BLAZER]

On 2005-08-24 17:59, Kyle wrote:

i don\'t think a rev limiter or an ignition problem would cause the tach to jump around...sounds like fuel surges.



when mine gets low on gas the fuel pressure gauge will drop down to like 3 then 2 then 1 and never does what his is doing unless it is being driving. (not like i have done that much http://www.s10v8.com/images/smilies/icon_biggrin.gif )

[MARKYMARK]

there is plenty of fuel inside a carb to rev to 9000 rpm. i\'ve done it with no fuel supply connected . what type of distributor are you running ? hei modules can start going bad and the coil pick will cause same symptoms. msd or mallory rarely have any problems.

[largekid]

its a mallory breakerless and msd blaster coil.. i have a fuel rail with a preasure guage attached, but the crappy autozone guage breaks super easy..

new plugs, new 8mm set of wires(the first ones did it too)

No msd..

[carkiller]

what did the old plugs look like?

[tracker]

just a thought but how about a restriction in your exhaust? do you still have the cat installed? just a thought but had one in the shop for 2 weeks and none of the techs figured that out till i finally said something. clogged up cat will do it.

[sorj]

If the fuel pressure is 7 to 9 when this happens, it aint fuel, pressure that is. Of course 7 is a little on the low side but is ok. it can still be the carb but the key is it backfiring? If it is, that is a sign for ignition loss. If bogging, check carb for flow. Make sure it aint restricted on fuel supply and it aint vapor locking when hot. You can have pressure but be vapor locked also. Heck, install a clear fuel filter between fuel pump and tank (nearest to the pump) and see if the fuel is going bye-bye. I just ran into this same problem with my old 72 Impala, the stariner was collasping around fuel sending unit inlet. Hope this helps, sorj

[largekid]

i put brand new plugs in when i first started the motor and kept same plugs...havent pulled them since...exhaust is straight from the headers to flowmasters so no cat. if i can get a good pressure gauge to read pressure to my carb i\'d be alot better...tommorrow is my work day for the truck lol

[MARKYMARK]

only other thing i could consider is a weak valve sping. possible do a compresion check that will give you a good base to start with.

[coppergmc]

I would start with the timing. I have seen the distributor keep advancing past 5000 rpms. If you have a timing light with the dial, check your timing while you rev the motor to 4500 to 5000. Just make sure that you aren\'t advancing too far. I had a mallory unilite that I recurved because it was doing the same thing.



For racing, I prefer mechanical advance. For the street, I use vacuum.

[carkiller]

also some aftermarket tachs have a rev limiter...are you sure your dont....



also do you have a vac advance hooked up. if you do, try unplugging it and see if it still does it.the vacumm port might be giving to much vac at high rpms.....

[largekid]

my tach dosen\'t have a rev limiter and i have plugged the vac advance period...it dosen\'t run very strong when its hooked up

[eigthmile]

I had a very similar problem recently. Drove me nuts. Ended up carb being way to rich. It would only start missing once boosters started giving fuel supply. Since you say it does it at 4500 try and just open the secondaries see where it starts missing at then. Also if you can bring it up to where it misses and see if boosters in carb are spraying a mist or more like a stream. Mine were spraying a weird looking stream switched carbs and problem solved. If you have a holley put it back to factory jets and powervalve. If you need to know the specs let me know what you have and I will get them for you.

[largekid]

uses vacuume secondarys, I cant control when they open..

the distributor has vacuumm advance but i plugged it.. its not backfiring..





AHHHHH

[eigthmile]

see what the boosters in the primary side are doing and get back with us. You can open the secondaries. Under vacuum reservoir is the arm for secondaries lift on it gently and see if it misses.

[riotpolice75]

first thing to look at would be a fuel filter in my opinion if you suspect a fuel problem. take it off and try to blow through it. if you cant or its hard then get a new one. second thing would be to determine if it is in fact fuel related. if it was me, id rev it up till it started to carry on, let it do it for about 5 seconds then have someone else kill the ignition while its carryin on. pull the plugs and see if the plugs read lean. another thing that came to mind was your ignition control module. the stock ones are no good and have a serious problem with scattering spark. also if your vacuum advance is plugged off then its not gonna advance the timing. that could do it too.







smith